QC scrim opacity depends on scroll position

Scrim opacity depends on scroll position after the initial
entrance animation.

This is the last CL needed for b/16683381. Also, this is the
last feature-requestish ui-tweak I plan on doing before
LMP (I promised this one to UX a while ago).

Bug: 16683381
Change-Id: I8a62ff1eda6f2174b50f71f54c4301c111182b7f