Use getActivityToken() instead of mToken in isTopOfTask

mToken could be local tokan and causes IllegalArgumentException.

bug: 29072444
Change-Id: I0f889a70a848ffea31663ed80337e1605c75223e

core/java/android/app/Activity.java

@@ -5952,7 +5952,7 @@ public class Activity extends ContextThemeWrapper
             return false;
         }
         try {
-            return ActivityManagerNative.getDefault().isTopOfTask(mToken);
+            return ActivityManagerNative.getDefault().isTopOfTask(getActivityToken());
         } catch (RemoteException e) {
             return false;
         }