drmserver: read locked ringtones

avc: denied { read } for comm="generic" path="/data/system_de/0/ringtones/ringtone_cache" dev="sda35" ino=1114120 scontext=u:r:drmserver:s0 tcontext=u:object_r:ringtone_file:s0 tclass=file

Change-Id: I40992733d779743be92c15a094d166a3df64a10f
Fixes: 30167454