Global app standby flag

Disable app standby features if app_standby_enabled
is set to 0. This allows for UI and experiments to
control the feature.

Bug: 70655630
Test: adb shell settings put global app_standby_enabled 0
      adb shell dumpsys usagestats
      adb shell am get-standby-bucket <packagename>

Change-Id: Id6c62b078e52040767100f2997832cc586bb0806