[Wi-Fi] Fix TalkBack focus lost in SavedAccessPointsWifiSettings2

PreferenceGroup in a controller is not in a RecyclerView,
TalkBack loses focus after PreferenceGroup#removeAll.

To fix it, only add/remove the preference if it's necessary.

Bug: 153278854
Test: make RunSettingsRoboTests ROBOTEST_FILTER=SavedAccessPointsPreferenceController2Test
Change-Id: Ie9ad5481c33865dce2b7db04d0f969b1f49b80e1