Make limitations of MPEG4Writer explicit

Make limitations of MPEG4Writer explicit

o No more than 2 tracks will be supported
o No more than one video and/or one audio tracks will be supported
o Only take video and/or audio track (for instance, no text tracks)
o If there is no track before start() is called, bail out.

At the same time, make sure the errors from addSource() report to
addTrack(), not to start().

Bug: 7991013

Change-Id: I1ca35aaeb75b5448d75ed2c6c10dd12ecea720ab