Move NDK compiler logic to a shared props file and import it in test and
build projects to unify Android NativeAOT and Mono builds.

- Set ASSETS_DIR for crypto tests
  (using same pattern as existing crypto test)
- Set AppContext.BaseDirectory

We should probably change crypto tests to use
just AppContext.BaseDirectory.

- Use ReferenceXUnitWrapperGenerator != false to define SINGLE_FILE_TEST_RUNNER (android & singlefile targets) instead of IsFunctionalTest
- Condition SingleFileTestRunner compile and TrimmerRootAssembly (TestUtilities) on wrapper/generator and SkipTestUtilitiesReference
- Add Android NativeAOT publish/trim settings and set NativeLib/CustomNativeMain for android nativeaot
- Make DisableImplicitFrameworkReferences respect preexisting value and add explicit DisableImplicitFrameworkReferences=false in nativeaot tests dir
- Import testing\tests.targets when TargetsAndroid is true
- Update many NativeAOT smoke tests Main signatures to public static int Main(string[] args) to match runner invocation

- Ensure they have Main entry point
- Tighten MSBuild conditions:
  - Only set NativeLib/CustomNativeMain for Android when OutputType == Exe
  - Only import testing/tests.targets for Android when using MicrosoftNETSdk and building executables
  - Make TestNativeAOT/UseNativeAOTRuntime defaults apply when TestBuildMode == nativeaot and only include monodroid-nativeaot.cs for executable/XUnit-wrapper cases

This reverts commit 4ed06ec.

- Avoid importing logic for SharedLibrary
- Fix library filename
- Set HasMergedInTests to ensure projects build

This reverts commit 37b7b4e.

This import is now covered by the IsMergedTestRunnerAssembly property.
Fixes a warning about a double import.
Fixes issue where mono runtime tests are getting incorrect assembly name
due to this undesired import.