[One .NET] set $(EnableSingleFileAnalyzer) by default

[jonathanpeppers]

Fixes: #5943
Context: https://github.com/dotnet/sdk/blob/c1b8e80be1f7cca767ed9f3c90a45d6830b166a0/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.Analyzers.targets#L92-L96

By default, we need to enable ILLink's "single file" analyzers for
Android app projects. We should also do this for Mono.Android.csproj
and Mono.Android.Export.csproj.

After enabling this analyzer, no warnings appeared. But I do see the
appropriate analyzers setup in projects now:

Analyzer
    ~\android-toolchain\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.CodeFixProvider.dll
        IsImplicitlyDefined = true
    ~\android-toolchain\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.RoslynAnalyzer.dll
        IsImplicitlyDefined = true

[jonpryor]

Pity I can't find any docs for $(EnableSingleFileAnalyzer)…

[jonathanpeppers]

Let's link to this PR as documentation? dotnet/sdk#15239

[jonpryor]

https://docs.microsoft.com/en-us/dotnet/core/deploying/single-file

[jonpryor]

Fixes: https://github.com/xamarin/xamarin-android/issues/5943

Context: https://github.com/dotnet/sdk/blob/c1b8e80be1f7cca767ed9f3c90a45d6830b166a0/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.Analyzers.targets#L92-L96
Context: https://docs.microsoft.com/en-us/dotnet/core/deploying/single-file
Context: https://github.com/dotnet/sdk/pull/15239

[.NET Single File Apps][0] are a .NET deployment construct in which
"all application-dependent files [are built] into a single binary."
It's conceptually similar to Xamarin.Android `.apk` files, and there
is some overlap in the "semantic [API incompatibilities][1]" between
"normal" desktop .NET apps and Xamarin.Android/single-file apps,
such as the (lack of) utility in `Assembly.Location`, which is the
empty string in Xamarin.Android apps when
`$(EmbedAssembliesIntoApk)`=True, which is required for "app store"
distribution.

For .NET 6+ app builds, enable analyzers which check for usage of
these "semantically changed" APIs, by setting
`$(EnableSingleFileAnalyzer)`=True.

Update `Mono.Android.csproj` and `Mono.Android.Export.csproj` to also
enable the single-file analyzers.

After enabling this analyzer, no warnings appeared. But I do see the
appropriate analyzers setup in projects now:

	Analyzer
	    ~\android-toolchain\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.CodeFixProvider.dll
	        IsImplicitlyDefined = true
	    ~\android-toolchain\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.RoslynAnalyzer.dll
	        IsImplicitlyDefined = true

[0]: https://docs.microsoft.com/en-us/dotnet/core/deploying/single-file
[1]: https://docs.microsoft.com/en-us/dotnet/core/deploying/single-file#api-incompatibility

[jonathanpeppers]

I had a typo in my original commit message:

--~\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.RoslynAnalyzer.dll
++~\android-toolchain\dotnet\sdk\6.0.100-preview.6.21280.2\Sdks\Microsoft.NET.Sdk\targets\..\analyzers\ILLink.RoslynAnalyzer.dll