sysui: refactor for extensibility.

Make mHost in QuickStatusBarHeader protected.

Bug: 28759287
Change-Id: Ifd71a5c1c62f0672addea02bbd4bdd832b85862b
(cherry picked from commit d1c4238aa5038aa78a0987099abb6179c0f169fe)

packages/SystemUI/src/com/android/systemui/statusbar/phone/QuickStatusBarHeader.java

@@ -92,7 +92,7 @@ public class QuickStatusBarHeader extends BaseStatusBarHeader implements
     private TouchAnimator mAlarmTranslation;
     protected TouchAnimator mSettingsAlpha;
     private float mExpansionAmount;
-    private QSTileHost mHost;
+    protected QSTileHost mHost;
 
     public QuickStatusBarHeader(Context context, AttributeSet attrs) {
         super(context, attrs);