Don't call setTask twice.

The method ActivityRecord.setTask() removes the ActivityRecord from
its old task's mActivities ArrayList. In jb-mr2 it did not have this
side effect (there was no mActivities) so calling it twice was not a
problem. This fix causes setTask to only be called once for the target
activity.

Fixes bug 11557835.

Change-Id: If2b6d4b297e86130009713efe6891a24fad3dd15