WidgetTray revamp work

- RecyclerView is rendering
- Animation is connected
- Drag and drop is now handled
- UI tweaking: background, margins, more to come.
- Flicker and preview not loading issue:
  fixed height for the horizontal scroll view.
- Shortcuts are added
- Widget Preview loading should support shortPress for drop
- UI tweaks left: overlay of arrow when there are more items to scroll
- icons are added in the section header
- Sorting of widget sections and widget horizontal list
- Adding all the padding constants to dimen.xml file
- RecyclerView should only support one view type

For items to be addressed in follow up patches OR CLs,
TODO is added to the comment.

b/19897708

Change-Id: Ibfc4da1696a23d20bada93db46e126706eb13cdc

src/com/android/launcher3/Insettable.java

@@ -18,6 +18,10 @@ package com.android.launcher3;
 
 import android.graphics.Rect;
 
+/**
+ * Allows the implementing {@link View} to not draw underneath system bars.
+ * e.g., notification bar on top and home key area on the bottom.
+ */
 public interface Insettable {
 
     void setInsets(Rect insets);