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

quickstep/src/com/android/launcher3/taskbar/TaskbarModelCallbacks.java

@@ -184,7 +184,6 @@ public class TaskbarModelCallbacks implements
         int predictionSize = mPredictedItems.size();
         int predictionNextIndex = 0;
 
-        boolean isHotseatEmpty = true;
         for (int i = 0; i < hotseatItemInfos.length; i++) {
             hotseatItemInfos[i] = mHotseatItems.get(i);
             if (hotseatItemInfos[i] == null && predictionNextIndex < predictionSize) {
@@ -192,13 +191,11 @@ public class TaskbarModelCallbacks implements
                 hotseatItemInfos[i].screenId = i;
                 predictionNextIndex++;
             }
-            if (hotseatItemInfos[i] != null) {
-                isHotseatEmpty = false;
-            }
         }
         hotseatItemInfos = mControllers.taskbarRecentAppsController
                 .updateHotseatItemInfos(hotseatItemInfos);
         mContainer.updateHotseatItems(hotseatItemInfos);
+        mControllers.taskbarViewController.updateIconsBackground();
     }
 
     @Override