Fix the availability of UseOpenWifiPreferenceController.

Right now, it crashes if mEnableUseWifiComponentName is disabled by the
open wifi app.

Bug: 150773571
Test: manual - along with ag/10741759, verified that disabling
mEnableUseWifiComponentName hides the toggle instead of causing crash.
Test: make RunSettingsRoboTests ROBOTEST_FILTER=UseOpenWifiPreferenceControllerTest

Change-Id: I47e36fab411e4e4d1a23b7c91504139f124222e1