[NativeAOT] MSBuild-related logic to get projects to build

[jonathanpeppers]

This is an initial step to get the NativeAOT build working the same as
iOS does. At a high level, the changes are:

• Run ILLink with our existing trimmer/linker pipeline.

• Temporarily change our RID to linux-bionic-arm64.

• Pass the output to ILC, the NativeAOT compiler.

• Turn the RID back to android-arm64.

A list of MSBuild/workload changes required so far:

• $(UseMonoRuntime) is false when $(PublishAot) is true

• Set $(_IsPublishing) as NativeAOT expects dotnet publish to be
  the only way NativeAOT is used. Setting this in our targets allows
  the proper *.NativeAOT.* packs to restore inside of a dotnet build.

• $(SelfContained)=true, seems to be required for the
  linux-bionic-arm64 runtime packs to restore. All Android apps are
  self-contained, so this might be fine.

• Create a new Microsoft.Android.Sdk.NativeAOT.targets file that
  uses $(_AndroidNdkDirectory) to set various properties for
  NativeAOT's build (ILC).

• HACK: set $(_targetOS) to linux to make the NativeAOT build
  attempt to run. --targetos:android results in a build error that
  says Android is not supported.

• Disable $(RunAOTCompilation), Mono's AOT compiler and the
  <RemoveRegisterAttribute/> MSBuild task.

• Disable $(AndroidEnableMarshalMethods) to be re-enabled in a
  future PR.

A list of blockers or things that don't work yet:

• NativeAOT apps won't launch yet (future changes for that)

• The NativeAOT cross compiler doesn't run on Windows:

Microsoft.NETCore.Native.Publish.targets(61,5): Cross-OS native compilation is not supported.
Microsoft.NETCore.Native.Unix.targets(296,5): error : Platform linker ('C:\Android\android-sdk\ndk\26.3.11579264\toolchains/llvm/prebuilt/windows-x86_64/bin/aarch64-linux-android21-clang++' or 'gcc') not found in PATH. Ensure you have all the required prerequisites documented at https://aka.ms/nativeaot-prerequisites.

• You can only target -r android-arm64 as there are no other
  linux-bionic-* runtime packs yet.

• -p:PublishAotUsingRuntimePack=true is currently required, but I
  will investigate if we should just set this by default.

Copilot reviewed 1 out of 6 changed files in this pull request and generated no suggestions.

Files not reviewed (5)

• src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets: Language not supported
• src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets: Language not supported
• src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets: Language not supported
• src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.targets: Language not supported
• src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets: Language not supported

Comments skipped due to low confidence (2)

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest2.cs:109

• [nitpick] The method name 'NativeAOT' is not descriptive. Consider renaming it to 'BuildNativeAOTApplication' or something similar to better reflect its purpose.

public void NativeAOT ()

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest2.cs:113

• [nitpick] The comment could be clearer. Consider rephrasing to: 'Ensure all required prerequisites are documented at https://aka.ms/nativeaot-prerequisites.'

// Ensure you have all the required prerequisites documented at https://aka.ms/nativeaot-prerequisites.

[dellis1972]

looks reasonable