Fix memory leak of DataSaverPreference

When DataSaverPreference is shown repeatedly, memory leak happens
since DataSaverBackend.Listener is not removed on onDetached().

TEST: Repeat enter and exit "Data usage" for
several times and check if the count of activity
is not increasing.

Bug: 72581115
Change-Id: Ic4a72de049bb87f1e522fe46dd0fb4bb58ed65a5

src/com/android/settings/datausage/DataSaverPreference.java

@@ -37,7 +37,7 @@ public class DataSaverPreference extends Preference implements DataSaverBackend.
     @Override
     public void onDetached() {
         super.onDetached();
-        mDataSaverBackend.addListener(this);
+        mDataSaverBackend.remListener(this);
     }
 
     @Override