stagefright: do not use default bitrates in MPEG4Writer

- this fails the muxer tests that verify that all metadata received
  from the extractor was muxed properly.
- instead, propagate bitrate to encoder output formats, so muxer
  will have this data during normal processing.

Bug: 29208170
Change-Id: If327d472e3607997840a8d601a453dbb7c625c95