GameManager: Add Public API

Add the public API to GameManager:
- getGameMode()

Bug: b/178860939
Test: atest CtsGameManagerTestCases
Test: atest GameManagerTests
Test: atest GameManagerServiceTests
Test: atest GameManagerServiceSettingsTests
Change-Id: Ie6030e4ec796c9ecf2c756f24f00e445572accda
11 files changed