Fix flickering issues with divider during split animation

This CL makes changes to the "split divider placeholder view", which was intended to cover up the backdrop a little during the split confirm animation. The placeholder view is now larger (fullscreen) and fades in with the animation movement, so there is no longer a period of time when it looks like an awkward rectangle.

New timings and interpolators confirmed with UX. Also renamed some variables and added comments for clarity.

Fixes: 344573331
Test: Manually verified that the visual bug no longer happens on large and small screen, and from desktop and Overview.
Flag: EXEMPT bugfix
Change-Id: I3b37f2b0478035d7a3181ae7c23962fe75a13b2c