Add toast error message for no WiFi on wireless debugging.

Show a toast to connect to a WiFi network when the user tries to
enable wireless debugging while not connected to any Wi-Fi network.

Bug: 150139061
Bug: 149843499

Test: make RunSettingsRoboTests ROBOTEST_FILTER=WirelessDebuggingPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WirelessDebuggingEnablerTest

Change-Id: I389f5fddc874f60e8156ecf6ef2f1c6eddc1d7a5
Exempt-From-Owner-Approval: LGTM