Skeletal implementation of a MediaTranscodeManager API with corresponding JNI layer.

This change is the first iteration of a new high-level media transcode API.
At this point the API is not hooked up to the underlying native media transcoding service.
More transcoding options will be added in following CLs.

Test: Ran MediaTranscodeManagerTest, verified call sequence using logcat. Flashed build
on blueline and verified basic media functionality.

Change-Id: I190e41b468265bd339d0fbca243db2a196e5a04e