Reroute Uri conversions though MediaProvider.

Upcoming changes will prevent apps from reading "_data" columns
directly, which is required to convert between DocumentsProvider and
MediaProvider Uris.  To solve this, delegate the call() through
MediaProvider, where it can perform the "_data" lookup on behalf
of the caller.

Also add new getMediaUri() call to offer symmetry.

Bug: 111960973, 117627072, 110961701
Test: atest android.provider.cts.MediaStoreUiTest
Change-Id: I53c640704d86047d7a4bf1702aca027873395abf