Improve handling of GameService when bound processes die.

 - Add callbacks to observer service lifecycle events to ServiceConnector.
 - Leverage callbacks to avoid re-binding to GameService in order to
   send disconnected event
 - Leverage callbacks to detect when GameSessionService process dies and
   destroy all GameSessions.

Test: Manually verified GameSession overlays are removed when
GameSession process crashes. Added unit test covering behavior.
Bug: 220204229
Bug: 215599171

Change-Id: Idda96f221e40c88f0f80c95bea94556275da5166