Allow system_server access to /data/media files passed via Binder.

Addresses denials such as:
 avc: denied { read } for comm="Binder_6" path="/data/media/0/zedge/ringtone/love_tone_2014-ringtone-1665292.mp3" dev="mmcblk0p28" ino=1534267 scontext=u:r:system_server:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file
 avc: denied { getattr } for comm="Binder_9" path="/data/media/0/zedge/ringtone/love_tone_2014-ringtone-1665292.mp3" dev="mmcblk0p28" ino=1534267 scontext=u:r:system_server:s0 tcontext=u:object_r:media_rw_data_file:s0 tclass=file

Change-Id: I5e5744eecf2cbd4fc584db8584be4e9101bcb60c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>