[Wi-Fi] Enhance Wifi Settings unit test cases.

Add new unit test cases in
1. WifiP2pSettingsTest.java
2. WifiConfigController2Test.java
3. WifiP2PPreferenceControllerTest.java
4. WifiP2pPeerTest.java

the coverage rate of
1. com.android.settings.wifi will raise from 44% to 45%
2. com.android.settings.wifi.P2p will raise from 69% to 85%

Bug: 151696220
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2pSettingsTest
      make RunSettingsRoboTests ROBOTEST_FILTER=WifiConfigController2Test
      make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2pPeerTest
      make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2PPreferenceControllerTest
Change-Id: I365494e1654376b23b08f18b1a799ffc59323a4c