Remove misleading error log.

It's perfectly ok for a secondary display to not have a home stack.
There is no reason to report it.

Related to b/17677973.

Change-Id: Ia9d52cabb601760d32d2b847dfa0ca4f304e4e2a

services/core/java/com/android/server/wm/DisplayContent.java

@@ -162,7 +162,7 @@ class DisplayContent {
     }
 
     TaskStack getHomeStack() {
-        if (mHomeStack == null) {
+        if (mHomeStack == null && mDisplayId == Display.DEFAULT_DISPLAY) {
             Slog.e(TAG, "getHomeStack: Returning null from this=" + this);
         }
         return mHomeStack;