[Wi-Fi] Fix AddAppNetworksFragmentTest one unit test failed case

onDestroy_quitWorkerThread in AddAppNetworksFragmentTest failed due to java.lang.IllegalStateException: no event down from INITIALIZED
Solution: Initialize fragment before calling the onDestroy API.

Bug: 151696220
Test: make RunSettingsRoboTests ROBOTEST_FILTER=AddAppNetworksFragmentTest
Change-Id: I80d28c6e7490a36a7ff26313d30d33b2f7d727c1