MediaCodec refactoring part 1-a: buffers become separate class

MediaCodecBuffer is meant to replace ABuffer and MediaBuffer in
communication between framework components. As the first step, replace
use of ABuffer in MediaCodec with MediaCodecBuffer, and adjust related
classes accordingly.

MediaCodec.getBuffer() and related APIs now returns MediaCodecBuffers;
thus change MediaCodec clients to use MediaCodecBuffer accordingly.

Test: manually tested for key use cases (Camera, YouTube and Play Movies)
Change-Id: Iba7ce131645e75ce5ddbf497fb793ab38b7f245b