Adjust some of the timings around animating / showing the dismiss gradient

Dan wanted longer + linear fade in of the dismiss gradient. Also wanted
fade out to match the timing of 'snap to edge'.

Test: Manual -- move PIP around, observe how gradient fades in / out
Bug: 35358628
Change-Id: I847d96e70184c0093e64a7e2e895a9c5f30fea8b