transcoding: Add HEVC->AVC transcoding test.

1) This CL verifies hevc to avc transcoding is working.
Still need to verify the transcoding result in the
coming CLs.
2) This CL also fix a bug that the track format
is not set when sending job to transcoding service.

Bug: 145628554
Bug: 154734285
Test: Unit test

Change-Id: Ia740e1e969357c3f719966c114f1e5182f4d7f66