Allow getDisplayContentLocked to return null...

... and check for null returns. This prevents DisplayContent objects
from containing null Display references.

Bug: 7368565 fixed.
Change-Id: I830fb4c1349204c366193657a95a92c48ccee66c
diff --git a/services/java/com/android/server/wm/DisplayContent.java b/services/java/com/android/server/wm/DisplayContent.java
index 3898ebc..68cdbfc 100644
--- a/services/java/com/android/server/wm/DisplayContent.java
+++ b/services/java/com/android/server/wm/DisplayContent.java
@@ -66,14 +66,17 @@
     int mBaseDisplayWidth = 0;
     int mBaseDisplayHeight = 0;
     int mBaseDisplayDensity = 0;
-    final DisplayInfo mDisplayInfo = new DisplayInfo();
-    final Display mDisplay;
+    private final DisplayInfo mDisplayInfo = new DisplayInfo();
+    private final Display mDisplay;
 
     // Accessed directly by all users.
     boolean layoutNeeded;
     int pendingLayoutChanges;
     final boolean isDefaultDisplay;
 
+    /**
+     * @param display May not be null.
+     */
     DisplayContent(Display display) {
         mDisplay = display;
         mDisplayId = display.getDisplayId();