Allow app to stop lockTaskMode if started by manifest attribute

It is possible lockTaskMode was started by the system process
because android:lockTaskMode is set to a locking value in the
application's manifest instead of the app calling
startLockTaskMode. In this case TaskRecord.mLockTaskUid will
be 0, so we compare the callingUid to the
TaskRecord.effectiveUid instead so the app can exit lockTaskMode.

Bug: 21464182
Change-Id: Ibca6de8e4b17051d5fcbb05cde9c8aefed7216f2