Fix launcher <-> taskbar dark themed icons background color

- Moved colors to final in TaskbarViewController (since controllers are
  recreated on theme change)
- Clarified some variable names
- Animate from progress 1 to 0 instead of vice versa, since 0
  corresponds to launcher's theme icon color which is what the alignment
  animation is going towards

Test: in gesture nav with dark themed icons in hotseat, go to overview
and ensure smooth transition to ligher themed background; return home
and ensure reverse transition
Flag: none
Fixes: 271651337

Change-Id: I4451f995a3e79e512c59a834051f601ead70ff87