Fix velocity reporting for negative values > -1

Currently, a velocity with a magnitude of (for example) 0.1 and a
negative sign will be displayed as positive 0.1 by the
FasterStringBuilder. This is because casting a float to an int makes it
zero, and the sign gets lost: (int)-0.1f = 0.
Add this sign here.

Bug: 112705257
Test: enabled pointer location, and manually generated some flings that
have negative direction and magnitude less than 1. Confirmed that the
velocity sign is displayed correctly.

Change-Id: I0d777e790d324739ce248d3e6b8e04692400d832