Observe NetworkCallback instead of polling

Switch from polling for the first connected WiFi network to observing
the currently connected WiFi network and associating that with the
preference at the time the preference is created or updated.

Whenever the ConnectedAccessPointPreference is removed or UI is stopped
unregister the callback.

Bug:68031656
Test: make RunSettingsRoboTests
Change-Id: I5d1ed83b6a13e8a83fae04bfdce8d0f13c2ba0ac