Better scanning of secondary storage volumes.

There's been a long-standing bug where secondary external storage
volumes were being scanned into a single "external" database.  We've
had the logic in MediaProvider for a long time to support individual
databases on a per-UUID basis, but that regressed at some point.

This change revives that logic, which has the benefit of greatly
speeding up media scanning of secondary storage devices, since we can
preserve those databases instead of churning the primary "external"
database whenever they're ejected and reinserted.

Future work will clean up stale databases in the recently added idle
maintenance service.

Bug: 117932814, 122060093
Test: atest MediaProviderTests
Test: atest cts/tests/tests/media/src/android/media/cts/MediaScanner*
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: Ib48577a2146758acd882db0b1767e88f3a70247f