Guard Mono-specific AOT targets for CoreCLR runtime, add XA1042 warning

[Copilot]

Setting <RunAOTCompilation>true</RunAOTCompilation> in a project file (e.g. Directory.Build.props) causes build failures when switching from Mono to CoreCLR, because MonoAOTCompiler runs unconditionally regardless of the active runtime.

Changes

• Microsoft.Android.Sdk.Aot.targets — Add '$(_AndroidRuntime)' == 'MonoVM' guard to the ImportGroup (MonoAOTCompiler SDK imports) and _AndroidAot target condition
• Microsoft.Android.Sdk.DefaultProperties.targets — After RunAOTCompilation defaults are resolved, detect Mono-specific properties (RunAOTCompilation, EnableLLVM) set with non-MonoVM runtimes, reset them to false, and set flags for warning emission
• Xamarin.Android.Common.targets — Emit new XA1042 warning when the flags are set
• Resources.resx / Resources.Designer.cs — New XA1042 message
• xa1042.md — Documentation for the new warning
• AotTests.cs — Two tests: RunAOTCompilationWithCoreClrWarnsAndSkipsMonoAot and EnableLLVMWithCoreClrWarnsAndIsIgnored

Example warning

warning XA1042: The MSBuild property 'RunAOTCompilation' is not compatible with the CoreCLR runtime and will be ignored.
Either remove the property or guard it with a condition: Condition="'$(UseMonoRuntime)' == 'true'"

Migration guidance

<!-- Before: breaks when switching to CoreCLR -->
<RunAOTCompilation>true</RunAOTCompilation>

<!-- After: only applies to Mono -->
<PropertyGroup Condition=" '$(UseMonoRuntime)' == 'true' ">
  <RunAOTCompilation>true</RunAOTCompilation>
</PropertyGroup>