Fix "square corners" appearance of new Back Arrow

This fixes the appearance of the "ENTRY" state, by setting an explicit
corner radius that matches the corners of the "ACTIVE" state.

While doing this, it turned out that the "INACTIVE" and "CANCELLED"
states were reusing the corner radius of the "ENTRY" state, so that
had to be split off into a new constant: "cancelledEdgeCornerRadius"
(this is the radius when the Back Arrow is "squished" against the edge,
 and if let go, will cancel the current gesture)

Bug: 238445578
Test: Manual
Change-Id: Ie5e7df298565f87c4262d79fa6b7960bf8e07977