Kill process if it's cached & idle and in forced-app-standby mode

This behavior could be turned OFF via the flag kill_fas_cached_idle
in device_config.

Bug: 152573287
Test: atest FrameworksServicesTests:ActivityManagerTest
Test: atest FrameworksMockingServicesTests:AppStateTrackerTest
Change-Id: Ifa6950582fbf6a24595b36163810f3c7e9345394