[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

src/com/android/settings/wifi/WifiConnectionPreferenceController.java

@@ -29,6 +29,8 @@ import android.os.SimpleClock;
 import android.os.SystemClock;
 
 import androidx.annotation.VisibleForTesting;
+import androidx.lifecycle.LifecycleObserver;
+import androidx.lifecycle.OnLifecycleEvent;
 import androidx.preference.PreferenceGroup;
 import androidx.preference.PreferenceScreen;
 
@@ -50,7 +52,7 @@ import java.time.ZoneOffset;
  * controller class when there is a wifi connection present.
  */
 public class WifiConnectionPreferenceController extends AbstractPreferenceController implements
-        WifiPickerTracker.WifiPickerTrackerCallback {
+        WifiPickerTracker.WifiPickerTrackerCallback, LifecycleObserver {
 
     private static final String TAG = "WifiConnPrefCtrl";
 
@@ -97,6 +99,7 @@ public class WifiConnectionPreferenceController extends AbstractPreferenceContro
             UpdateListener updateListener, String preferenceGroupKey, int order,
             int metricsCategory) {
         super(context);
+        lifecycle.addObserver(this);
         mUpdateListener = updateListener;
         mPreferenceGroupKey = preferenceGroupKey;
         this.order = order;
@@ -124,6 +127,14 @@ public class WifiConnectionPreferenceController extends AbstractPreferenceContro
                 this);
     }
 
+    /**
+     * This event is triggered when users click back button at 'Network & internet'.
+     */
+    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
+    public void onDestroy() {
+        mWorkerThread.quit();
+    }
+
     @Override
     public boolean isAvailable() {
         return mWifiPickerTracker.getConnectedWifiEntry() != null;