Permit mediaextractor its own file source for apk and ringtone files.

Previously we fall back to the mediaserver file source.
This does not affect behavior as the fallback works fine; however,
the log messages may cause confusion.

    [73402.683908] type=1400 audit(1491338955.878:121): avc: denied { read }
    for pid=18381 comm="generic"
    path="/data/system_de/0/ringtones/alarm_alert_cache" dev="sda35"
    ino=2490374 scontext=u:r:mediaextractor:s0
    tcontext=u:object_r:ringtone_file:s0 tclass=file permissive=0
    [73402.683932] type=1400 audit(1491338955.884:122): avc: denied { read }
    for pid=18383 comm="generic"
    path="/data/system_de/0/ringtones/ringtone_cache" dev="sda35"
    ino=2490376 scontext=u:r:mediaextractor:s0
    tcontext=u:object_r:ringtone_file:s0 tclass=file permissive=0

Test: Ringtone and CTS
Bug: 37500781

Change-Id: Ie6d8e6d2b7301d00957733f173aeebbe9d0d1998