Acquire service lock in AppTaskImpl.moveToFront()

Oversight from previous refactor in O release done in ag/2987716

Change-Id: I2e2d6e55251eb319ddee0db97eedc1c574736ead
Merged-In: I2e2d6e55251eb319ddee0db97eedc1c574736ead
Fixes: 111336376
Test: builds

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

@@ -98,7 +98,7 @@ class AppTaskImpl extends IAppTask.Stub {
         final int callingUid = Binder.getCallingUid();
         final long origId = Binder.clearCallingIdentity();
         try {
-            synchronized (this) {
+            synchronized (mService) {
                 mService.mStackSupervisor.startActivityFromRecents(callingPid, callingUid, mTaskId,
                         null);
             }