Allow Game Service to trigger the Game Session creation

With this change, the GameServiceProviderInstanceImpl no longer
automatically creates a game session every time a new game task is
started. Instead, the GameServiceProviderInstanceImpl calls the new
GameService#onGameStarted method with details about the game task that
just started.

The GameService can implement onGameStarted with appropriate logic to
determine whether a game session should be created. If the service
determines that a session should be created, then the GameService can
call GameService#createGameSession.

After calling onGameStarted, the GameServiceProviderInstanceImpl will
maintain a GameSessionRecord in the NO_GAME_SESSION_REQUESTED state.
Only games with a record in this state will have a session created in
response to createGameSession being called. When createGameSession
is called for a record in this state, the record will transition to the
GAME_SESSION_REQUESTED state. Once the game session is finally created
and attached to the record, it will switch to the GAME_SESSION_ATTACHED
state.

Test: Manual e2e testing and unit tests
Bug: 207035150
Bug: 202414447
Bug: 202417255
CTS-Coverage-Bug: 206128693
Change-Id: I799f8410da6936ab242fa925d77b0276875cc20f

Change-Id: Id6da59fd3a5951ba5f032c825d1f1f28d3320965