Prevent abuse of MediaRoute2ProviderService#notifyRequestFailed()

The method can be called with a random requestId, or called multiple
times with the valid requestId. It can result unnecessary calls
to MediaRouter2Manager#onRequestFailed().

This CL resolves this problem.

Bug: 157873556
Test: Passes CTS and mediaroutertest
Change-Id: Ia7dd4ddafd532b9f23bf19491a0820d5a74f6a70