Add SystemMediaRoute2Provider2

This is a no-op refactor in preparation for the upcoming logical
changes.

SystemMediaRoute2Provider2 is intended to manage all system routing
including:
- system-provided routes (for example, BT, wired headsets).
- app-provided routes (for example, Cast mirroring).

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases CtsMediaHostTestCasts
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I184cbe5d612736fec0d925cad6b12c76e6f4f686