Allow stat/read of /data/media files by app domains.

Resolves denials such as:
avc:  denied  { read } for  pid=23862 comm="Binder_4" path="/data/media/0/DCIM/.thumbnails/1390499643135.jpg" dev="mmcblk0p28" ino=171695 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file

avc:  denied  { getattr } for  pid=26800 comm="ImageLoader" path="/data/media/0/DCIM/.thumbnails/1390499643135.jpg" dev="mmcblk0p28" ino=171695 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file

Change-Id: I8221359123ecc41ea28e4fcbce4912b42a6510f0
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>