Codec2: separate platform dependency

- Move C2* objects out of android namespace
  - keep some stubs in place to not break dependencies
- Separate usage flags from gralloc usage flags
- Remove some unused classes

Bug: 64121714
Test: Build
Change-Id: Ibd0a5c2b4c3835a46aaa9c75c467bc728c2d85eb
25 files changed