Change MEDIA_TYPE through Files table.

As part of implementing strongly-typed storage in the Q release, we've
needed to limit the items visible in strongly-typed views.  (For
example, the Images view must only include images.)

This change fixes a place inside the OS that was implicitly relying on
update() leaking between these strong data types.  If we're changing
the media type of an already-scanned file, we need to do that through
the Files table first, and then we can update the details at the
strongly-typed Uri.

Bug: 112467162
Test: atest android.media.cts.MediaScannerTest
Change-Id: I61c8b62e04f6542882745a20e9aed96275427b5f