Calculate swipe time proportional to the swipe distance in millimeters.

In the framework, gesture detection is performed based on millimeters.
This CL lets CTS use millimeters for swip time calculation so that any
device reliably recognizes a slow swipe as a touch exploration.

Bug: 183915071
Test: android.accessibilityservice.cts.TouchExplorerTest on chromebooks (hatch/kled, eve), chromebox (puff/kaisa), and phone (crosshatch)
Change-Id: If65fc279f02f74a881acb97a7209b41b15408256