Show correct Wi-Fi status when switching to multi window mode

In WifiSettings, when switching to multi window mode,
WifiSettings fragment is recreated and "onPause()" is called,
then WifiTracker stops and wrong Wi-Fi status is shown.

Move "WifiTracker.startTracking()" to "onStart()",
 "WifiTracker.stopTracking()" to "onStop()"
to make sure to show correct Wi-Fi status.

Bug: 36078677
Test: Manual test OK

Change-Id: I215f180b1082f6039c03de8b4a3ca27d1f364c75