[Wi-Fi] Refine code for saved AccessPoints

SubscribedAccessPointsPreferenceController extends from
SavedAccessPointsPreferenceController to remove duplicate
code.

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: Ia494117ae98f043ce65b2f9f6e4b4daccc8df543