[release/11.0.1xx] Keep generated AAPT2 rules in R8 inputs on incremental builds - #12974

## Summary
- Register the merged `aapt_rules.txt` as `ProguardConfiguration` and `FileWrites` in `_CreateBaseApk`, whose item groups are evaluated even when the target is up to date. Previously `_CreateBaseApkWithAapt2` was not invoked on an incremental build, dropping AAPT2 keep rules from R8's configuration.
- Compute the rules path in `_CreateBaseApkInputs` so the AAPT2 link task and the consuming target use the same path.
- Add an integration test with `AndroidLinkTool=r8` and `TrimMode=full` that failed before the fix and passes on an initial build and two successive incremental builds that rerun R8 without relinking resources. It also verifies incremental cleanup preserves the rules file.

## Validation
- Full `Microsoft.Android.slnx` build succeeded with the repository's pinned SDK.
- Targeted regression test: 1 passed after the change (failed before it).
- Existing AAPT2 link tests: 2 passed.
- Rebuilt SDK package contains the updated targets.

This reproduces and fixes the dropped-rules incremental path; the issue's intermittent clean-CI trigger was not independently reproduced.

Related to #12942

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