Add GameManager service.

GameManager service is a service to manage game related features. This
patch creates the initial service to manage the game mode and persist
the data across reboot.

Bug: b/174956354
Test: atest GameManagerTests
Test: atest GameManagerServiceTests
Test: atest GameManagerServiceSettingsTests
Change-Id: Iee2ab47c79d29f48847bc6d4b3eb781a5af18ff4