Adjust down event location when two-finger dragging in TouchExplorer

The bug is happened because the slope between the down event and the
move event of the swipe gesture dispatched from TouchExplorer is
not smooth. So we also ajust the down event position as the moving
event. And we will update the corrupted CTS test case later.

Bug: 152931647
Test: none
Change-Id: I4d4b1ff2975f440dad0babb87d0af1b6ce348f93