Create Espresso test for WifiSettings.

This method was being stripped by proguard and hence the tests could not
run.

Bug: b/36403696
Test:  runtest --path
packages/apps/Settings/tests/app/src/com/android/settings/wifi/WifiSettingsUiTest.java

Change-Id: I30b26e960a876c0c024154bee809c0ab6e988e39