Address ghost spots that show up after touchscreen is re-enabled

When touches are cancelled as a result of a touchscreen being disabled,
the pointers that were down when the touchscreen was disabled show up as
touch spots briefly while a new pointer goes down after the touchscreen
is enabled. Remove these ghost touches.

Bug: 216718625
Test: manual, see bug
Change-Id: I4fa381da9ca0e285dff4b405cad644fcea840d9e