Move PreviewBackground into its own class file.

This is done in preparation for new PreviewBackground animation
where we will be animating in the first page preview items when
closing the Folder from a non-first page.

Bug: 36022592
Bug: 35064148
Change-Id: Id4d54da16ddfa852ca639a2dc8d328307d4d443a

src/com/android/launcher3/folder/FolderAnimationManager.java

@@ -57,7 +57,7 @@ public class FolderAnimationManager {
     private GradientDrawable mFolderBackground;
 
     private FolderIcon mFolderIcon;
-    private FolderIcon.PreviewBackground mPreviewBackground;
+    private PreviewBackground mPreviewBackground;
 
     private Context mContext;
     private Launcher mLauncher;