[Wi-Fi] Refine code for saved AccessPoints

The change targets to refine SavedAccessPointsPreferenceController and
SubscribedAccessPointsPreferenceController

1. Remove feature flag check for saved Passpoint display.
2. They do not implements WifiManager.ActionListener.
3. Separate the code to collect saved AccessPoints data and the code to
   update preferences.
4. Let getAvailabilityStatus() returns result according to the saved
   AccessPoints data.
5. Update preferences at SavedAccessPointsWifiSettings#onStart()
   instead of life cycle onStart() of these 2 controllers.
   This is for the change of 2.
6. Do not use PreferenceGroupChildrenCache in
   SubscribedAccessPointsPreferenceController, this is for code
   alignment of SavedAccessPointsPreferenceController.
   Now these 2 controllers have very similar design, it will be easier
   to refine SubscribedAccessPointsPreferenceController as a child of
   SavedAccessPointsPreferenceController.

Bug: 127206629
Test: make RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.wifi.savedaccesspoints
      Manual: Add and forget Wi-Fi accesspoints and observe UI display.

Change-Id: Id6f95d5b64e57a98c2c2e6ab09a4dec70a71b615