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/FastBitmapDrawable.java

@@ -32,7 +32,7 @@ import android.graphics.Rect;
 import android.graphics.drawable.Drawable;
 import android.util.SparseArray;
 
-class FastBitmapDrawable extends Drawable {
+public class FastBitmapDrawable extends Drawable {
 
     static final TimeInterpolator CLICK_FEEDBACK_INTERPOLATOR = new TimeInterpolator() {
 
@@ -72,7 +72,7 @@ class FastBitmapDrawable extends Drawable {
     private boolean mPressed = false;
     private ObjectAnimator mPressedAnimator;
 
-    FastBitmapDrawable(Bitmap b) {
+    public FastBitmapDrawable(Bitmap b) {
         mAlpha = 255;
         mBitmap = b;
         setBounds(0, 0, b.getWidth(), b.getHeight());