[Audio Focus] Observer should use request state

The AudioFocusObserver should use AudioFocusRequestState
instead of separate parameters for OnFocusGained/Lost. This
was not the case before because AFRS was added after the
observer.

We need this to expose the request id to clients so they
can get a MediaController that will only control a specific
session.

BUG=892771

Change-Id: I4f6e2e60de6dbc82f779ffbfec3d91628208c42d
Reviewed-on: https://chromium-review.googlesource.com/c/1363684
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616916}
8 files changed