Refactor the OutputMediaItemList to a proxy class

This CL has no logic changes. It only move the "List<MediaItem>
mOutputMediaItemList" from the member field of MediaSwitchingController
to a helper class "OutputMediaItemListProxy". In the next CL, we
will flag guard the new implementation of updating the output
media item list to fix the existing exception crashes.

Flag: EXEMPT refactor
Bug: b/400241830
Test: Manual test and presubmit
Change-Id: Icff47b16035283d56adb9fde07203a01b62d2cc0