Fix fling gestures

InputHandlerProxy::HandleGestureFling assumes that the timestamp on
the fling gesture is in the same system as base::TimeTicks. By giving
it our smaller relative event timestamps (most often taken from a
QElapsedTimer) this ends up here being calculated as a negative time
on the fling animation curve.

Fix the issue by applying a delta at the ui::TouchEvent conversion so
that the resulting GestureFlingStart has a timestamp that can be
reliably interpreted.

Change-Id: I5e149bf64887305119643359495f89be6e0c3ffb
Reviewed-by: Andras Becsi <andras.becsi@digia.com>