Add Universal Game Mode Hint mode switching to GameManagerService.

Bug: 214405108

Test: atest com.android.server.app.GameManagerServiceTests
Test: Manual with "adb logcat | grep GameManagerService", and opening/backgrounding a game on the attached device.
Change-Id: I0883e6bae316199aeeab9f7dda8934ccd67e7243
Merged-In: I0883e6bae316199aeeab9f7dda8934ccd67e7243