Fix the Robolectric Tests Failures

- The ShadowWifiManager class is not working as expected
  - The getSystemService(WifiManager.class) return null when testing

- Use Mockito class instead of Shadow class

Bug: 214906101
Bug: 214938188
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=AllInOneTetherSettingsTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherSettingsTest

Change-Id: Ic54af8524fbff1b6ac916ca8e1bcac52f413663a