Cleanup the activity before the task and stack

The method cleanupActivityLocked() should be called before
removeActivityFromHistoryLocked(). Previously it didn't matter but
now that we null TaskRecord.stack when removing the last activity
from the task it does.

Fixes NPE in bug 19491381.

Change-Id: Ia6f8f560c0fe4df330577f313e24d7dd02c18409
1 file changed