Animating this new property allows us to adjust the scrim without
competing with other state animations that could be setting the
scrim progress. Also reset scrimMultipler = 1 for state transitions.
Test: Swipe from home to overview, then during that transition swipe
from overview to home; when the former transition ends while the
second is still in progress, ensure scrim value doesn't jump
Bug: 144170434
Change-Id: I44b57ced1c6902e558fe1818bcbf11ceb4ff2f6f
Tony Wickham
b176dc99b2