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/widget/WidgetImageView.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2011 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.launcher3.widget;
+
+import android.content.Context;
+import android.graphics.Canvas;
+import android.util.AttributeSet;
+import android.widget.ImageView;
+
+public class WidgetImageView extends ImageView {
+    public boolean mAllowRequestLayout = true;
+
+    public WidgetImageView(Context context, AttributeSet attrs) {
+        super(context, attrs);
+    }
+
+    public void requestLayout() {
+        if (mAllowRequestLayout) {
+            super.requestLayout();
+        }
+    }
+
+    @Override
+    protected void onDraw(Canvas canvas) {
+        canvas.save();
+        canvas.clipRect(getScrollX() + getPaddingLeft(),
+                getScrollY() + getPaddingTop(),
+                getScrollX() + getRight() - getLeft() - getPaddingRight(),
+                getScrollY() + getBottom() - getTop() - getPaddingBottom());
+
+        super.onDraw(canvas);
+        canvas.restore();
+    }
+}