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

drmserver.te

@@ -41,6 +41,7 @@ r_dir_file(drmserver, media_rw_data_file)
 # Read resources from open apk files passed over Binder.
 allow drmserver apk_data_file:file { read getattr };
 allow drmserver asec_apk_file:file { read getattr };
+allow drmserver ringtone_file:file { read getattr };
 
 # Read /data/data/com.android.providers.telephony files passed over Binder.
 allow drmserver radio_data_file:file { read getattr };