MediaCodec refactoring part 1-b: secure buffers

- Define SecureBuffer to encapsulate secure buffers containing
decrypted data.
- ACodec and MediaCodec use SecureBuffer to communicate between
themselves.

Bug: 32133435
Test: Play an encrypted content in Play Movies
Change-Id: Ic70f7f78aad15bd91ab8ccd31c340bc7964ab1b3