fixed crash during animation to all apps

- proguard was stripping getter/setter that was
  used only by reflection
diff --git a/proguard.flags b/proguard.flags
index c91ca67..607c14b 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -7,6 +7,11 @@
   public void onClickAllAppsButton(android.view.View);
 }
 
+-keep class com.android.launcher2.CellLayout {
+  public float getDimmedBitmapAlpha();
+  public void setDimmedBitmapAlpha(float);
+}
+
 -keep class com.android.launcher2.AllApps3D$Defines {
   *;
 }