Remove dock divider surface when it's not visible.

We achieve the removal by notifying System UI about the visibility of
the dock divider. This way System UI can change visibility of the root
view, which in turn will cause the WMS to destroy or create the surface
as necessary.

Bug: 25844096
Bug: 25683717

Change-Id: Idbc33368db697a059af49106dfadb80c3d7d06c1
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 57338be..568b601 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -1877,6 +1877,11 @@
     <permission android:name="android.permission.MANAGE_APP_TOKENS"
         android:protectionLevel="signature" />
 
+    <!-- Allows System UI to register listeners for events from Window Manager.
+         @hide -->
+    <permission android:name="android.permission.REGISTER_WINDOW_MANAGER_LISTENERS"
+        android:protectionLevel="signature" />
+
     <!-- @hide Allows the application to temporarily freeze the screen for a
          full-screen transition. -->
     <permission android:name="android.permission.FREEZE_SCREEN"