Stop splitting mouse events.

Mouse events should always be dispatched to the window under the cursor,
so it shouldn't be split in any case.

This change's parent already uses mouse cursor position to dispatch
mouse events regardless of the coordinate of events. Therefore this
change is just to make it conceptually correct.

Bug: 134788085
Test: The window under the cursor always responds to the gesture.
Change-Id: Id0d791bb75df7be8ba5c49e840b5655c008fded0