Fix a longstanding bug in the shade pulldown trajectory.

Two parts to this:

1. We were looking at the measured height of the close view
   for a lot of our computations, which---particularly with
   recent changes that dispense with the old 3-window
   implementation---is increasingly unreliable

2. We were overestimating the minY that the panel must be
   pulled down before animation starts. It was enforced
   jankiness!

Bug: 5359178
Change-Id: Ie09b62226b7b0977527348b5527478665ece1df8