Add dynamic item in output switcher if it is available

-Add extra item for dynamic group if getSelectedMediaDevice() > 1
-Move common method to base class

Bug: 171455839
Test: atest MediaOutputAdapterTest MediaOutputControllerTest MediaOutputBaseDialogTest MediaOutputDialogTest MediaOutputGroupAdapterTest MediaOutputGroupDialogTest
Merged-In: I5990df864cb7c5795e2d1a1cd404807d265b4cc3
Change-Id: I5990df864cb7c5795e2d1a1cd404807d265b4cc3