[Ravenwood] Cleanup RATR implementation and project structure

- Create a dedicated RATR implementation specifically for device-side
  tests in junit-stub-src so we won't need to worry about device-side
  compatibility when updating RATR.
- Enable Ravenizer on RavenwoodBivalentTest to ensure the "stub"
  RATR is working as intended
- Because the real RATR is now in junit-impl-src, a lot of classes no
  longer need to be in junit-stub-src, as junit-src is now a lot cleaner
- Remove all Ravenwood checks in RATR since the real one will always
  only run on Ravenwood tests
- Remove RATRHook and move the hook callback methods directly in RATR

Flag: EXEMPT host test change only
Bug: 356918135
Test: atest RavenwoodBivalentTest_device_ravenizer
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Change-Id: I18577373833d8f6390bc685c23b857be65b904dc