Structure MediaProvider as an APEX.

Based on guidance from the Mainline team, we're placing the
MediaProvider APK inside a new APEX, as this will allow us to
move MediaStore.java inside the module boundary in a future CL.

As part of this restructuring, we need the pre-Mainline APK to
remain intact to preserve the user's media database.  There is
already logic in place from previous CLs that will migrate data
from the pre-Mainline to the post-Mainline world, including tests
to verify that the migration works.

Bug: 144247087
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I571eccbb2950a7c518a682eae3350ba7ef9bd804