GameManagerService: Restrict get/set Game Mode to Games

Restrict the GameManagerService's getGameMode() and setGameMode()
functionality to only work with applications that are categorized as
games.

Bug: b/178860939
Test: atest GameManagerTests
Test: atest GameManagerServiceTests
Test: atest GameManagerServiceSettingsTests
Test: atest CtsGameManagerTestCases
Change-Id: Iaf42b995db3ef2a4040a5bf46dc327a952d4aa88