Fling to close tasks in the GestureNav can be too small.

The problem is that the velocity threshold to activate the fling
was set using pixels per second, so a better aproach would is to
use dp/s. So now there is a variable set in the dimes.xml file
called base_swift_detector_fling_release_velocity.

Test: Manually tested
Fix: 201252634
Change-Id: Ief14f25de136dead74f03cb24d2120b67900239e