[NativeAOT] improve build logic, part 2

[jonathanpeppers]

This is another set of changes to Android apps to run under NativeAOT:

• Default to $(TrimMode)=Full for NativeAOT, this enables trimmer warnings and should be the default mode for NativeAOT.

• Ensure _PrepareLinking MSBuild target runs at the appropriate time. This was causing Android.App.Activity's GetOnCreate_Landroid_os_Bundle_Handler() to get trimmed away.

• Default to $(LinkerFlavor)=lld to avoid a java.lang.UnsatisfiedLinkError at runtime.

• Include libc++_shared.so to avoid a java.lang.UnsatisfiedLinkError at runtime.

• Emit JavaPeerStyle.JavaInterop1 java stubs for NativeAOT, as it is easier to get working in place of JavaPeerStyle.XAJavaInterop1.

I updated our existing NativeAOT() test to assert for these changes where possible.

[jonathanpeppers]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jonathanpeppers]

There is one failure, that is probably network-related:

Mono.Android.NET_Tests, Xamarin.Android.NetTests.AndroidMessageHandlerTests.ServerCertificateCustomValidationCallback_Redirects / Release
Expected: 2
But was:  1

I think we can ignore, as these changes shouldn't affect it.

[jonpryor]

Draft commit message:

Context: 8d7713071c40766651664de25b53eb15225785d0
Context: https://github.com/dotnet/android/pull/9630

We're exploring how to get .NET for Android apps to build and run
using [Native AOT][0].

Default to `$(TrimMode)=Full` for NativeAOT, this enables trimmer
warnings and should be the default mode for NativeAOT.

Ensure the `_PrepareLinking` MSBuild target runs at the appropriate
time.  Failure to do so was causing
`Android.App.Activity.GetOnCreate_Landroid_os_Bundle_Handler()` to be
trimmed away.

Various fixes to avoid `java.lang.UnsatisfiedLinkError` errors:

  * Set `$(LinkerFlavor)=lld` by default to avoid:

        E AndroidRuntime: java.lang.UnsatisfiedLinkError: dlopen failed: cannot locate symbol "__start___modules" referenced by
          "/data/app/~~_ggpMC4foLk_jUUycm0CfA==/net.dot.hellonativeaot-fvszIWroqgweLHYgULxVoQ==/split_config.arm64_v8a.apk!/lib/arm64-v8a/libNativeAOT.so"...

  * Include `libc++_shared.so` within the app, as Native AOT output
    requires C++:

        E AndroidRuntime: java.lang.UnsatisfiedLinkError: dlopen failed: library "libc++_shared.so" not found: needed by
          /data/app/~~_W0B9EE3hhajnFvCHyUKSg==/net.dot.hellonativeaot-zlXemqHdkbHaLu60oYPVQQ==/lib/arm64/libNativeAOT.so in namespace clns-6

Emit `JavaPeerStyle.JavaInterop1` java stubs for NativeAOT, as it is
easier to get working in place of `JavaPeerStyle.XAJavaInterop1`.
Specifically, we need to be able to avoid P/Invokes related to
typemaps/etc.  XAJavaInterop1 hits `JNIEnvInit.RegisterJniNatives()`,
which is full of P/Invokes such as `TypeManager.GetClassName()`,
while `JavaInterop1` hits `ManagedPeer.RegisterNativeMembers()` which
goes through `JniRuntime` abstractions, allowing for a P/Invoke-free
code path.

I updated `BuildTest2.NativeAOT()` to assert for these changes where
possible.

[0]: https://learn.microsoft.com/dotnet/core/deploying/native-aot/