Make mirroring sessions releasable

Makes Output Switcher enable the Stop Casting button while mirroring.

Bug: b/396396416
Test: atest android.media.router.cts.SystemMediaRoutingTest
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I867e0a9c767b5182819d5dd6db4f780492f8e312