transcoding: Rework mediatranscoding test.

Create a sperate directory for all tests associated with
mediatranscoding.
This CL is a preparation of hooking with actual service,
adding perf benchamrk, power measurement.

Bug: 145628554
Test: MediaTranscodingTest.

Change-Id: Ifcdb26eae2e71599e2e19bb92b991f88809ec5e0