Send correct downtime for motion events in split pointers case

Currently, for split pointer scenario, the down time is still the
the down time for the first pointer on the display. This might not
necessarily be the downTime for a window in split pointer case.
Need to send the correct downTime when we split pointers.

Test: atest inputflinger_tests
Bug: 228331266
Bug: 220109830
Change-Id: I80cf889e3243c39619ba96bd2b74073642cabc36