MediaCodec refactoring part 1-c: buffer ownership

- Buffers are (roughly) owned by themselves.
- As a corollary, remove output format change related events
and replace by inspecting formats associated with each buffers.

Bug: 32133435
Test: gts-tradefed run gts -m GtsExoPlayerTestCases
Test: (manual) Run Play Movies app to play a secure content.
Change-Id: I6b57da61c2d71acd0d5be4281de823ba1c95b72f