Partially exempt headless system apps from app standby.

The user can't interact with headless system apps (pre-installed apps
without any activities) but they're expected to work properly. We don't
want to fully exempt the apps from app standby, but they should be fine
operating in the ACTIVE bucket, so we make sure that headless system
apps never fall below the ACTIVE bucket.

Bug: 155761007
Test: atest FrameworksServicesTests:AppIdleHistoryTests
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: I1549bb81eca293be31691b079bab2142cbcdf8a7