Mock WifiManager.class for Roboletric test-cases

Bug: 214938188
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=AddNetworkFragmentTest
make RunSettingsRoboTests ROBOTEST_FILTER=ConfigureWifiEntryFragmentTest
make RunSettingsRoboTests ROBOTEST_FILTER=ContextualWifiSliceTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiConfigControllerTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiSliceTest

Change-Id: I96873c48b36ed11b30e32bba1b98b6328ad17bb7