Add proper permission check and multi-user handling to GameManagerService

Add the following permissions which are used by GameManagerService:
- permission.MANAGE_GAME_MODE

Additionally, enforce multi-user handling with
ActivityManager.handleIncomingUser().

Bug: b/178111358
Test: atest GameManagerTests
Test: atest GameManagerServiceTests
Test: atest GameManagerServiceSettingsTests
Test: atest PermissionPolicyTest
Change-Id: Ib503a8f50746d4079d32bd0515b6a9c771b6792f