Fix NPE when running ActivityManagerServiceTest

Since ATM is not passed to WindowProcessController, NPE happened when trying to access mAtm.mGlobalLock
Add a null check to prevent this kind of NPE.

Test: atest ActivityManagerServiceTest
Change-Id: I813386adcd6eab5206a75c8bf56b0c26968b26c8
Fixes: 111651991
diff --git a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index 86c5d4d..6832f0d 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -639,7 +639,8 @@
         }
     }
 
-    ActivityTaskManagerService(Context context) {
+    @VisibleForTesting(visibility = VisibleForTesting.Visibility.PACKAGE)
+    public ActivityTaskManagerService(Context context) {
         mContext = context;
         mFactoryTest = FactoryTest.getMode();
         mSystemThread = ActivityThread.currentActivityThread();