Split VOLUME_EXTERNAL and VOLUME_PRIMARY.

To let developers focus on specific concrete storage devices in Q,
we need a volume name that can be used to point at the primary
external storage device.  We had been using VOLUME_EXTERNAL for that,
but we've heard that certain apps are making deep assumptions that
media item IDs are globally unique across all volumes.

Thus these changes merge all volumes back into a single underlying
database, and VOLUME_EXTERNAL works with all of the currently
attached volumes.  The new VOLUME_PRIMARY name can be used to focus
on the primary storage device when desired.

When developers try inserting items directly into VOLUME_EXTERNAL,
we gracefully assume they meant VOLUME_PRIMARY.

Bug: 128451765
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I682ff6e9aaab4f5315a46c9825313a438548c7e6