[Wi-Fi] Fix thread leak in WifiConnectionPreferenceController

Stop the worker thread for WifiPickerTracker at onDestroy
event. The event will be triggered when users click back button
at 'Network & internet' page.

Bug: 161659380
Test: manual
      1. Launch Settings 'Network & internet'.
      2. Use below command to observe the thread.
         ps -T SettingsProcessId | grep WifiConnPrefCtr
      3. Click back button and use the command again,
         the thread should not exist.
Change-Id: I9145d534ade168beca52524e7cf85d87e2feb1a0