Allow Phone to write cached ringtones.

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: I120a69ac4f58c64db6f169ae4f9942ce357b0b1f