Fix NPE when scheduling transaction for null client

Application process was crash and the application thread was set
to null when obtaining the client transaction object. NPE was
thrown by using it to schedule transaction.

Bug: 130006936
Test: WindowProcessControllerTests
Test: ActivityStartControllerTests
Test: RootActivityContainerTests

Change-Id: I65ecb124e014e5d80ebc283884475619f95239fa

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

@@ -98,7 +98,7 @@ public class WindowProcessController extends ConfigurationContainer<Configuratio
     private final ActivityTaskManagerService mAtm;
     // The actual proc...  may be null only if 'persistent' is true (in which case we are in the
     // process of launching the app)
-    private volatile IApplicationThread mThread;
+    private IApplicationThread mThread;
     // Currently desired scheduling class
     private volatile int mCurSchedGroup;
     // Currently computed process state
@@ -192,8 +192,11 @@ public class WindowProcessController extends ConfigurationContainer<Configuratio
         return mPid;
     }
 
+    @HotPath(caller = HotPath.PROCESS_CHANGE)
     public void setThread(IApplicationThread thread) {
-        mThread = thread;
+        synchronized (mAtm.mGlobalLockWithoutBoost) {
+            mThread = thread;
+        }
     }
 
     IApplicationThread getThread() {