[Xamarin.Android.Build.Tasks] DTBs and _ResolveLibraryProjectImports

[jonathanpeppers]

I spent a little time looking at build logs of design-time builds
inside VS Windows. To trigger these, I was editing an .axml file and
saving it.

I found some perf issues...

The first thing I noticed was that _ResolveLibraryProjectImports
runs on every DTB:

Building target "_ResolveLibraryProjectImports" completely.
Input file "obj\Debug\100\designtime\build.props" is newer than output file "obj\Debug\100\stamp\_ResolveLibraryProjectImports.stamp".

Then I looked at how build.props was actually changing between
different targets such as:

• UpdateGeneratedFiles - the general DTB target
• SetupDependenciesForDesigner - the Android designer runs this
  target

Comparing the files, I saw this difference:

- AndroidSdkPath = C:\Program Files (x86)\Android\android-sdk
+ AndroidSdkPath = C:\Program Files (x86)\Android\android-sdk\
- AndroidNdkPath = C:\Program Files (x86)\Android\android-sdk\ndk-bundle
+ AndroidNdkPath = C:\Program Files (x86)\Android\android-sdk\ndk-bundle\
- JavaSdkPath = C:\Program Files\Android\Jdk\microsoft_dist_openjdk_1.8.0.25
+ JavaSdkPath = C:\Program Files\Android\Jdk\microsoft_dist_openjdk_1.8.0.25\

So in some cases _ResolveMonoAndroidSdks is running to normalize the
paths and other times not... _CreatePropertiesCache should depend on
_ResolveMonoAndroidSdks to fix this issue.

After that I found the next issue:

- DesignTimeBuild = True
+ DesignTimeBuild = true

What??? It appears that the casing differs depending on which part of
the IDE is triggering the build. I suspect the designer is using one
casing while VS proper uses another.

I think we should call ToLowerInvariant() on every line in the
build.props file to work around MSBuild's case insensitivity.

This change saves ~350ms on every design-time build. However, if
build.props changes all the time it may do even better, since
build.props triggers almost all MSBuild targets in Xamarin.Android.