Implement controller to disable transcoding cache

This is to provide users with control over whether to disable
transcoding cache or not.

By default we keep the transcoding cache enabled.

The controller resides within the Media transcoding settings under
Developer Options.

Test: Manual. Unit tests added.
Bug: 185246580
Change-Id: I5cc85e4a254c87b5c84c32a303e8294c918748c2