MediaMuxer: Remove SAMPLE_FLAG_SYNC flag

MediaMuxer.writeSampleData() uses MediaCodec.BufferInfo
objects to specify the flags, so use directly the MediaCodec
flags.  This is more intuitive and avoids users of the
MediaMuxer API having to create translations between
MediaMuxer's and MediaCodec's flag constants.

Change-Id: I18e12a6839ffb2c63da62516a4dc4c94e8bb3d17
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 9169479