Fix MediaRoute2ProviderService TODOs

This CL ignores requests that contain unknown session IDs and
make sure sequence of binder calls are aligned with notifySession
Created/Updated/Released.

Bug: 157873546
Bug: 157873487
Test: Run CTS and atest mediarouter test
Change-Id: Ie31e2fca8160093cf54886180e67d40865a35d6d