Support dynamic feature flag config

For CTS testing purposes, have the DeviceConfig feature flag control
whether app hibernation behavior is stubbed rather than whether the
service itself is started. This allows us to change the feature flag in
tests without restarting system_server

Bug: 175830282
Test: atest AppHibernationServiceTest
Change-Id: Id9ea16345cac84a069ff53939cef1564a585c466