"Disallow adjust volume" restricted dialog didn't show

root casue: VolumeSeekBarPreference & SeekBarPreference will return
false when call isSelectable() then this preference won't perform Click

Fixes: 123745455
Test: Manual & robotest
Change-Id: Iba27b503e8a272260c80bd41aecdd5cdbc39f962

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

@@ -80,11 +80,6 @@ public class VolumeSeekBarPreference extends SeekBarPreference {
         mAudioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
     }
 
-    @Override
-    public boolean isSelectable() {
-        return false;
-    }
-
     public void setStream(int stream) {
         mStream = stream;
         setMax(mAudioManager.getStreamMaxVolume(mStream));