Close AirplaneModeEnabler once lifecycle classes are destroyed.

The AirplaneModeEnabler is a ContentObserver, and it must be closed when
the object containing it is deleted. Otherwise, it will continue to
exist and cause a memory leak.

Test: atest -c SettingsUnitTest
Bug: 177265744

Change-Id: Idd48b601be1133ee02010e23d8dcb394d1e29bd3

src/com/android/settings/network/NetworkProviderSettings.java

@@ -459,6 +459,12 @@ public class NetworkProviderSettings extends RestrictedSettingsFragment
         super.onStop();
     }
 
+    @Override
+    public void onDestroy() {
+        mAirplaneModeEnabler.close();
+        super.onDestroy();
+    }
+
     @Override
     public void onActivityResult(int requestCode, int resultCode, Intent data) {
         super.onActivityResult(requestCode, resultCode, data);