Adjust event location when two-finger dragging in TouchExplorer

To scroll on the screen when using TalkBack, a user need to dragging two
finger pointers. If the dragging pointer are closer that a given distance
we use the location of the primary one. Otherwise, we should adjust the
event location to take the middle between the two finger pointers.

Bug: 126354276
Test: atest TouchExplorerTest
Change-Id: I9e95ccec964d7930498febdd02731b94826fbb7b