Fix UI glitches in the refreshed design.

 - Prevent group control from flickering by returning a stable value
 from `MediaOutputAdapter#getItemId`. The flicker was caused by the
 RecyclerView's add/remove animation.
 - Make the background of the items in the list to be non-transparent.
 This prevents visual overlap of RecyclerView elements whenever item
 dimension changes.

Bug: 406707984
Fix: 406707984
Flag: com.android.media.flags.enable_output_switcher_redesign
Test:  atest SystemUiRoboTests:MediaOutputAdapterTest,
On a physical device.

Change-Id: Id8e6353623f9bdeafab09d999be7590358efc02f