Yell when no home app was found.

If we didn't find a home app, the boot just appears to be wedged
and it's hard to investigate what the root-cause is.  Let's log to
make it more obvious.

Bug: 28465914
Change-Id: I9697af83eb27efb54f1f703506d4f148c96425b0

services/core/java/com/android/server/am/ActivityManagerService.java

@@ -3789,6 +3789,8 @@ public final class ActivityManagerService extends ActivityManagerNative
                 intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                 mActivityStarter.startHomeActivityLocked(intent, aInfo, reason);
             }
+        } else {
+            Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
         }
 
         return true;