Call MediaStore#scanFile directly in RingtoneManager

Reproduced the failing case, even without thread pool changes.
Since RingtoneManager was blocking on the call anyways, changing it
to use the synchronous scanFile method directly should be fine.

Bug: 131882699

Test: manual add MP3 with RingtonePickerActivity

Change-Id: I53b49ed17403cda93ecfefb6985133b161a96983