Add initial support for GameSessions and corresponding unit tests.

 - Define GameSession & GameSessionService Game Service provider SPIs.
 - Implement support for binding to GameSessionService, creating
   GameSessions, and destroying GameSessions.
 - Refactor Game Service provider selection into
   GameServiceProviderSelectorImpl. Parse and validate the
   declared GameSessionService associated with the GameService.

Test: atest FramworksMockingServicesTests and manual e2e
Bug: 204504879
Bug: 202414447
Bug: 202417255
CTS-Coverage-Bug: 206128693
Change-Id: I301579d5c7426a55c442b7ae04aacbedf293a4d6