transcoding: Address API council comments Part1

1. Create a base class TranscodingRequest for VideoTranscodingRequest
and upcoming ImageTranscodingRequest in Androd T. Also have builder in each type
with VideoTranscodingRequest.builder inherit TranscodingRequest.builder.
2. Hiding the PRIORITY_REALTIME and setPrority api for Android S.
3. Hiding TRANSCODING_TYPE_VIDEO as it is only for internal bookkeeping
4. Move TRANSCODING_TYPE_* and PRIORITY_* to inside TranscodingRequest.
5. Remove the setSrcUri, setDstUri, setVideoTrackFormat and set them in constructor.

Bug: 181551684
Test: atest  CtsMediaTranscodingTestCases:MediaTranscodeManagerTest
Change-Id: Ib6fead5e9fcb48bfd1db35f9f35402d6fa1ed73d