Transcoder: change src/dst to fd

MediaTranscoder takes fd as input from upper level service.

Also, leave file delete to client, since service side can't do file
create/delete. File opening is done by a callback into the app;
it seems delete should just handled by app side itself.

Minor refactor to reduce header pollution when included.

bug: 154734285
bug: 156003955, 152091443, 155918341
test: transcoder unit tests.

Change-Id: I4eb8a7d9fea2ccb479f09888353ac4e65bac16f8