Fix SysUI crash when locking while showing the Media Output Group dialog

This CL fixes a bug introduced by http://ag/15663086: When locking the
phone while showing the Media Output Group dialog, SystemUI would crash
because of a mismatch between MediaOutputAdapter.currentActivePosition
and MediaOutputController.mMediaDevices. That mismatch came from the
sharing of the controller between the 2 dialogs, that are now shown at
the same time (even though one is hidden).

Starting from this CL, both dialogs now use a separate instance of
MediaOutputController to avoid this kind of race condition. This CL also
adds a safeguard in MediaOutputBaseDialog to ensure that we notify an
item change within bounds, which would otherwise throw and crash.

Bug: 200781817
Test: Show media output group dialog then lock phone.
Change-Id: I7749cc963a5e145467b32aa3a9efb1e80009f324