MediaTranscodingService: Separate jni file into a sperate library.

This is needed when we move MediaTranscodeManager into apex.
Also rework the MediaTranscodeManager test so it could be invoked with
instrument.

Bug: 145628554
Test: MediaTranscodeManagerTest.
Change-Id: If0572af30d3aa7b7189ea0fc89582b83dd0d5d35
6 files changed