Introduced the visual stability manager

Notifications used to roam around and reorder whenever
they wished. Those little beasts must be tamed, hence
a new visual stability manager is introduced that
dictates the terms of their interplay.

Test: manual: add heads-up and see if they correctly appear
Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/notification/VisualStabilityManagerTest.java
Bug: 32442500
Change-Id: I8d7596fa7c14e0df68459a77d445f618d517ad51
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
index 2c8339a..f25e599 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationGroupManager.java
@@ -23,6 +23,7 @@
 import com.android.systemui.statusbar.NotificationData;
 import com.android.systemui.statusbar.StatusBarState;
 import com.android.systemui.statusbar.policy.HeadsUpManager;
+import com.android.systemui.statusbar.policy.OnHeadsUpChangedListener;
 
 import java.io.FileDescriptor;
 import java.io.PrintWriter;
@@ -35,7 +36,7 @@
 /**
  * A class to handle notifications and their corresponding groups.
  */
-public class NotificationGroupManager implements HeadsUpManager.OnHeadsUpChangedListener {
+public class NotificationGroupManager implements OnHeadsUpChangedListener {
 
     private final HashMap<String, NotificationGroup> mGroupMap = new HashMap<>();
     private OnGroupChangeListener mListener;