[monodroid] set MONO_AOT_MODE_NONE when AOT is off

[jonathanpeppers]

I was profiling an app's startup in Debug mode, and I saw it
printing a lot of stuff:

11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/data/user/0/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/data/user/0/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/storage/emulated/0/../legacy/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/storage/emulated/0/../legacy/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/data/app/com.xamarin.android.helloworld-F-gJdEOFfesdG4KjFIuCsQ==/lib/arm64/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/data/app/com.xamarin.android.helloworld-F-gJdEOFfesdG4KjFIuCsQ==/lib/arm64/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 D Mono    : AOT: image '/storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/HelloWorld.dll.so' not found: (null)
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/data/user/0/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/data/user/0/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/storage/emulated/0/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/storage/emulated/0/../legacy/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/storage/emulated/0/../legacy/Android/data/com.xamarin.android.helloworld/files/.__override__/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Trying to load shared library '/data/app/com.xamarin.android.helloworld-F-gJdEOFfesdG4KjFIuCsQ==/lib/arm64/libaot-HelloWorld.dll.so'
11-22 15:01:30.994  5616  5616 I monodroid-assembly: Shared library '/data/app/com.xamarin.android.helloworld-F-gJdEOFfesdG4KjFIuCsQ==/lib/arm64/libaot-HelloWorld.dll.so' not found
11-22 15:01:30.994  5616  5616 D Mono    : AOT: image '/Users/builder/jenkins/workspace/archive-mono/2019-08/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/HelloWorld.dll.so' not found: (null)

It seems like we are probing 25 locations for the AOT'd assembly, and
AOT is not even on at all?

I saw some issues on the MSBuild side as well as runtime:

1. By default $(AndroidAotMode) is blank, and at build time we emit a true value for application_config.uses_mono_aot in this case.
2. At runtime, we don't ever call mono_jit_set_aot_mode (MonoAotMode::MONO_AOT_MODE_NONE); and Mono seems to default to AOT being enabled.

So a few fixes:

1. Let's default $(AndroidAotMode) to None if AOT is off to be more explicit.
2. application_config.uses_mono_aot will no longer be true when AOT is off.
3. I fixed up the runtime to be sure mono_jit_set_aot_mode is always called.

Now I no longer get any of these AOT-related log messages.

The results are not dramatic, but still something:

Before:
11-22 15:01:14.122  5346  5346 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:181::786164
11-22 15:01:22.907  5406  5406 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:172::393454
11-22 15:01:24.902  5460  5460 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:169::927569
11-22 15:01:27.080  5511  5511 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:171::83402
11-22 15:01:29.061  5568  5568 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:172::116997
11-22 15:01:30.983  5616  5616 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:169::82048
After:
11-22 14:52:54.753  4605  4605 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:178::299914
11-22 14:53:02.152  4707  4707 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::750746
11-22 14:53:05.063  4764  4764 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::530538
11-22 14:53:07.395  4819  4819 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:165::721214
11-22 14:53:09.678  4878  4878 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:169::361579
11-22 14:53:11.921  4931  4931 I monodroid-timing: Runtime.init: end, total time; elapsed: 0s:166::757309

It seems like this might save ~10ms on startup for non-AOT apps.

I also removed a random string androidPackageName = null;
declaration that is unused.