Move selected items collapsing logic inside MediaSwitchingController.

- This is needed because the operation should be performed on the
output device list, not the merged output and input device list.
- It wasn't done initially because MediaSwitchingController was used
for the TV Output Switcher. After ag/33429645 this is no longer the
case.

Bug: 432652209
Test: atest MediaOutputAdapterTest MediaSwitchingControllerTest
Flag: EXEMPT refactor
Change-Id: I23ca1080f5a6a27e8f746ea456b5034877e3a55a