stagefright: add test for MediaCodec::reclaim/release race

- Minor MediaCodec refactoring to allow dependency injection
- Introduce MediaTestHelper to access private methods
  from the test code

Bug: 154678891
Test: atest mediacodecTest
Merged-In: I14c23eeabc288b43ad2be6ded0f8858ce22844c4
Change-Id: I14c23eeabc288b43ad2be6ded0f8858ce22844c4
8 files changed