Allow MediaExtractor to create FileSource

W Binder:538_2: type=1400 audit(0.0:9): avc: denied { getattr } for path="/data/media/0/Qtc88.mp4" dev="dm-0" ino=678654 scontext=u:r:mediaextractor:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file permissive=0

W generic : type=1400 audit(0.0:9): avc: denied { read } for path="/data/media/0/Qtc88.mp4" dev="dm-0" ino=678654 scontext=u:r:mediaextractor:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file permissive=0

Test: Photos, Play Music, Play Movies, Youtube
Bug: 29125703
Change-Id: If84ab43b934944abf4c416db751ab6694835df83