Wait until any background events are processed.

Some MediaProvider events (such as thumbnail invalidation) are
handled on a background thread.  To reduce flaky tests, wait until
all background events have been processed, which is more robust
than sleep.

Bug: 134879702
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: Ia032b19c1b0e5c0096c55bf3d46a5153525e89fa