Expand our test coverage.

Now that we're starting to get test coverage metrics, let's get some
additional testing in place to improve them.

Remove onOpen() logic, as all physical external storage devices
are now placed inside a single "external.db" SQLite database for
the moment.

Bug: 142561358
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I020cdda7358fecdb68e34943a831771723d5f9f4
9 files changed