Don't always intercept swipe transitions

This CL improves the interception logic of SceneGestureHandler so that
the started position is taken into account when deciding whether a
current transition should be intercepted rather than interrupted. See
b/322291138 for details on use-case.

This CL also removes the condition that the current swipe transition
must be animating (i.e. the user must have moved their finger up) for it
to be interceptable.

Bug: 322291138
Test: atest SceneGestureHandlerTest
Flag: N/A
Change-Id: I317241e544f2a244af5cf6610f17ca18e6e321b9