Only check X velocity for max stick-to-target velocity.

This was designed so you can drag objects through the dismiss target from left to right (or vv) without them sticking. Checking the hypot of velocity only results in false negatives when dragging down to the target.

Test: manual
Bug: 157180669
Change-Id: I21e3c048c00202fc0660aaec0c22f7825bf26f46