Add the GameService skeleton and corresponding unit tests.

This change defines a basic GameService SPI and hooks into the GameManagerService in order to drive the start/stop APIs.

Test: atest FramworksMockingServicesTests:GameServiceManagerTests and
manaul e2e testing
Bug: 204504879
Bug: 202414447
Bug: 202417255
CTS-Coverage-Bug: 206128693
Change-Id: Ibda56cb0c023a307f83eae4091c5f63b02be339f