Cached ringtone files should be mlstrustedobject.

Both appdomain and priv_app can set the default ringtones, so the
cache files need to be mlstrustedobject.

avc: denied { write } for path="/data/system_de/0/ringtones/ringtone_cache" dev="mmcblk0p44" ino=1602501 scontext=u:r:priv_app:s0:c512,c768 tcontext=u:object_r:ringtone_file:s0 tclass=file permissive=0

Bug: 27366059
Change-Id: Ib362f58b180a62bd46800083d6c538426f955b10