Handle MEDIA_ROUTING_CONTROL runtime revocation

When the app op permission is revoked, MR2SI must unregister the
relevant manager records and invalidate the matching proxy router
instances.

We need to add a new listener so that apps get notified of a proxy
router invalidation and know when to stop using it. The alternative
approach of listening directly to app-op permission changes from
AppOpsManager would create a race condition between the permission
change and the invalidation + cleanup of the instance, leading to
undefined behaviour.

getInstance(Context, String) was promoted from system to public API for
Android V ag/25997373, but the promotion becomes unnecessary with the new
getInstance() overload. Reverting getInstance(Context, String) back to
SystemAPI.

Bug: 326387864
Test: atest CtsMediaBetterTogetherTestCases
Change-Id: I417eb09cd08a3b61388c69f1a33a4116b399d61d