Launcher performance optimizations: shaving about 3ms off Workspace.dispatchDraw()

-> Caching values of getChildOffset() and getChildRelativeOffset()
    -> Using fast invalidate pattern in workspace#screenscrolled
    -> eliminated updateAdjacentPageAlpha, combined with screenScrolled
       code where appropriate to take advantage of fast invalidate
    -> removed a bit of dead code (just for cleanups sake)

Change-Id: Id91542ff846d9495a0370707d02d1f647dab22d4

src/com/android/launcher2/AppsCustomizePagedView.java

@@ -1152,8 +1152,6 @@ public class AppsCustomizePagedView extends PagedViewWithDraggableItems implemen
         layout.createHardwareLayer();
 
         invalidate();
-        forceUpdateAdjacentPagesAlpha();
-
         /* TEMPORARILY DISABLE HOLOGRAPHIC ICONS
         if (mFadeInAdjacentScreens) {
             prepareGenerateHoloOutlinesTask(data.page, data.items, data.generatedImages);