Informing app idle listeners on enabled state changes.

Currently, if AppStandbyController is enabled after some listeners have
already registered, the listeners will be told that the system is in a
state of parole even though AppStandbyController thinks it's not in
parole. This change informs AppIdleStateChangeListeners when
AppStandbyController's enabled state changes as well so that they can be
in the correct state.

I also removed the call to setAppIdleEnabled() in onBootPhase() since
updateSettings() is called right afterwards and that also calls
setAppIdleEnabled().

Bug: 112329453
Test: atest AppStandbyControllerTests
also check logs to confirm that NetworkPolicyManagerService and
JobSchedulerService's listeners switch from ON to OFF when the system is
ready.

Change-Id: I88bc293c70b459f54f75f92126ad306d6ab8d9b7
(Cherry-picked from 3d64c041)