Fix overscroll detection on watch.

The overscroll detection doesn't work well when user scrolls diagonally
at the edge. E.g. when user swipes right when the view top is at top,
the swipe was considered overscroll if the finger has a very minor delta
with upwards direction.

This change fixes this issue by determining the swipe direction based on
the larger delta value between x and y axis.

Flag: EXEMPT small bug fix
Bug: 350679840
Test: atest FullScreenMagnificationGestureHandlerTest
Change-Id: I75f9b50157c621ba5a602cecef03c1dbb7ab41ad