Implement SystemMediaRoute2Provider2#release

Releases system media routing (aka mirroring) resources associated with
the given session. This is relevant for the "Stop Casting" button on the
Output Switcher.

Bug: b/396396416
Test: CtsMediaRouterTestCases
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I69dc6f4f316aa3b3841e7f6b8f5ddce5e7c2643b