[r8] Add configurable private-member obfuscation and optimization - #12668

Use AndroidR8ObfuscationMode as the sole opt-in, place its behavior-preserving default with the shared R8 defaults, and keep runtime-specific derivation and validation late. This prepares the branch to add runtime-remapping to the mode contract from PR #12668 after it merges.

Co-authored-by: Copilot App <[email protected]>

## Summary

Backports the complete relevant selective R8 obfuscation/optimization work from #12668 to .NET 10 servicing and incorporates the follow-up validation changes from #12753.

- keeps `$(AndroidR8ObfuscationMode)` defaulted to `disabled` on .NET 10, preserving existing behavior
- makes `private-members` available as an explicit opt-in, enabling R8 optimization while preserving Java class/interface and public/protected member names
- adds `XA1050` validation, English resources, documentation, and message documentation for unsupported values
- gates mode validation on `$(AndroidLinkTool) == r8`, so unrelated builds are unaffected
- adds task, packaging, incremental-build, invalid-mode, R8-only validation, and device coverage
- installs both Android default R8 configuration variants

## Release-branch adaptations

The release branch predates main's NativeAOT `R8Mapping` and user-Java-source rooting infrastructure. Device coverage is therefore adapted to MonoVM and CoreCLR, reads `mapping.txt` directly, and supplies the explicit keep rule needed for the Java probe. The release task's existing `string[]` configuration-file shape is retained.

No `.apkdesc` baselines are changed: unlike main, the .NET 10 servicing default remains `disabled`, and measured targeted packaging/device coverage did not require release baseline updates. No unrelated Java.Interop version changes are included.

## Validation

- built `Xamarin.Android.sln` and configured the repository local Android workload
- built `Xamarin.Android.Build.Tests` and `MSBuildDeviceIntegration`
- R8 task and packaging matrix: 9/9 passed, covering default, explicit `disabled`, explicit `private-members`, incremental builds, metadata/optimization state, invalid-mode `XA1050`, and validation with R8 enabled/disabled
- physical-device R8 coverage: 2/2 passed on MonoVM and CoreCLR
- changed XML files parse successfully
- `git diff --check` passes
- final review reported no findings

Backport of #12668; includes #12753.

Co-authored-by: Copilot App <[email protected]>

## Description

Follow-up to #12668 and review feedback on #12752.

Ports `_ValidateAndroidR8ObfuscationMode` to `main` and gates it on `$(AndroidLinkTool) == 'r8'`. Invalid `AndroidR8ObfuscationMode` values report XA1050 only when R8 is selected, while non-R8 builds remain unaffected. This preserves the behavior and `private-members` default introduced by #12668.

## Tests

- `dotnet-local.cmd build src\Xamarin.Android.Build.Tasks\Tests\Xamarin.Android.Build.Tests\Xamarin.Android.Build.Tests.csproj -v:minimal --no-restore`
- `dotnet-local.cmd test bin\TestDebug\net10.0\Xamarin.Android.Build.Tests.dll --filter "Name~InvalidR8ObfuscationModeIsValidatedOnlyForR8" -v:minimal`
  - invalid mode without R8 succeeds
  - invalid mode with R8 fails with XA1050
- XML parsing for `Xamarin.Android.Common.targets` and `Resources.resx`
- `git diff --check`

## Summary

Remove the experimental build-time managed-assembly rewriting approach for R8 so any future assembly-rewriting design can start from a clean foundation.

This cleanup is related to #12535 and the original prototype in #12575.

## What this PR undoes

This removes the managed rewriting implementation developed across the R8 obfuscation stack:

- #12629 added the PE metadata rebuild substrate.
- #12630 added managed JNI metadata rewriting from R8 mappings.
- #12631 added rewriting for generated trimmable type-map assemblies.
- #12632 and #12634 integrated and tested that rewriting approach for CoreCLR and NativeAOT.

Concretely, this PR removes the rewrite task, rewrite-only metadata/IL utilities, dedicated tests and fixtures, and the XA4325/XA4326 resources and documentation. The intent is to abandon this implementation rather than preserve an unused rewriting stack that a future design would need to work around.

## What remains in place

- The generic `R8Mapping` parser introduced in #12628 remains, along with its tests and the `MSBuildDeviceIntegration` consumer. It is independently useful for reading R8 mapping files.
- The ordinary R8 configuration and `private-members` obfuscation/optimization policy from #12668 remain unchanged. This PR does **not** disable R8 or remove private-member obfuscation.
- Existing D8/R8 packaging, ProGuard rule handling, and non-rewriting build behavior remain unchanged.
- Runtime remapping remains active as the stacked follow-up work in #12847, #12848, #12692, and #12844. Those PRs implement the alternative opt-in strategy without managed assembly rewriting and are not part of this cleanup diff.

We may revisit managed assembly rewriting in .NET 12 based on customer feedback and performance data, but with a fresh design rather than this implementation.

## Validation

- Built `Xamarin.Android.Build.Tasks`
- Ran `Microsoft.Android.Build.Tasks.Tests`
- Ran `Microsoft.Android.Sdk.TrimmableTypeMap.IntegrationTests`
- Built `MSBuildDeviceIntegration`
- Ran focused `R8MappingTests`