Volume can not be updated in Sound&notification

After "onActivityPause()" is invoked and "onActivityResume()"
is invoked again, "mVolumizer" is not empty object,
so volume is not updated.

To fix this issue, set "mVolumizer" is null in "onActivityPause()",
if "onActivityResume()" is invoked again, recreate "mVolumizer"
and update volume.

Bug: 37558283

Change-Id: Id91064ea32305b5bf203f34e31988ac923beee6d

src/com/android/settings/notification/VolumeSeekBarPreference.java

@@ -88,9 +88,10 @@ public class VolumeSeekBarPreference extends SeekBarPreference {
         mStopped = true;
         if (mVolumizer != null) {
             mVolumizer.stop();
+            mVolumizer = null;
         }
     }
-    
+
     @Override
     public void onBindViewHolder(PreferenceViewHolder view) {
         super.onBindViewHolder(view);