Small optimizations for the All Apps transition

- don't set properties in animations if they don't
change
- don't recalculate visible pages on every frame
- don't require scroll indicator to be an
ImageView

Change-Id: I099b29e677da2d0e401f677d47cdb8b5df51224e

src/com/android/launcher2/CellLayout.java

@@ -916,8 +916,10 @@ public class CellLayout extends ViewGroup {
     }
 
     public void setBackgroundAlpha(float alpha) {
-        mBackgroundAlpha = alpha;
-        invalidate();
+        if (mBackgroundAlpha != alpha) {
+            mBackgroundAlpha = alpha;
+            invalidate();
+        }
     }
 
     // Need to return true to let the view system know we know how to handle alpha-- this is
@@ -928,6 +930,7 @@ public class CellLayout extends ViewGroup {
         return true;
     }
 
+    @Override
     public void setAlpha(float alpha) {
         setChildrenAlpha(alpha);
         super.setAlpha(alpha);