[Wi-Fi] Enhance savedaccesspoints2 unit test cases

Add new unit test cases in
1. SavedAccessPointsPreferenceController2Test.java
2. SavedAccessPointsWifiSettings2Test.java
3. SubscribedAccessPointsPreferenceController2Test.java

The coverage rate of com.android.settings.wifi.savedaccesspoints2 will raise from 32% to 85%

Bug: 151696220
Test: Run following test commands
1. make RunSettingsRoboTests ROBOTEST_FILTER=SavedAccessPointsPreferenceController2Test
2. make RunSettingsRoboTests ROBOTEST_FILTER=SavedAccessPointsWifiSettings2Test
3. make RunSettingsRoboTests ROBOTEST_FILTER=SubscribedAccessPointsPreferenceController2Test

Change-Id: I43e43c95019659d507ae0d8b167c5d65cd036b95