Exception in the touch explorer when dragging.

1. During a drag in touch exploration we have two pointers moving in the same
   direction but inject only one of them. If the dragging pointer goes up we
   send an up to the view system and wait for all pointers to go up to transition
   to touch exploring state. At this point the dragging pointer id is cleared
   and if a new pointer goes down we are trying to send up (rather do nothing)
   for the dragging pointer which we already did and due to the invalid pointer
   id we get an exception when splitting the motion event.

bug:7282053

Change-Id: I690bf8bdf6e2e5851ee46a322c4a1bb7d484b53a