Added RootWindowContainer

RootWindowContainer will be the root/head of the window hierarchy.
Also, moved all access of DisplayContent into RootWindowContainer.

Bug: 30060889
Test: Manual testing and existing tests still pass.
Change-Id: Icdf80a4c053578fb91221e6f2b672e197054a6bf
diff --git a/services/core/java/com/android/server/wm/WallpaperController.java b/services/core/java/com/android/server/wm/WallpaperController.java
index b269d4c..58439eb 100644
--- a/services/core/java/com/android/server/wm/WallpaperController.java
+++ b/services/core/java/com/android/server/wm/WallpaperController.java
@@ -641,7 +641,7 @@
     }
 
     boolean adjustWallpaperWindows() {
-        mService.mWindowPlacerLocked.mWallpaperMayChange = false;
+        mService.mRoot.mWallpaperMayChange = false;
 
         final WindowList windows = mService.getDefaultWindowListLocked();
         // First find top-most window that has asked to be on top of the wallpaper;