Folder support in Taskbar

- Add TaskbarActivityContext which allows shared Launcher elements to
  "just work" using existing generic ActivityContext.
- TaskbarContainerView extends BaseDragLayer<TaskbarActivityContext>.
- Inflate FolderIcon and Folder using TaskbarActivityContext to be
  shown in TaskbarContainerView.
- Use TaskbarActivityContext's DeviceProfile to determine icon size
  instead of overriding in styles. This also ensures that normal
  BubbleTextView icons have the same size as FolderIcons.

Test: Place a folder in home screen hotseat, ensure it shows up in
taskbar and can be opened, and that apps inside it can be launched
or dragged.
Bug: 171917176

Change-Id: Ic25d2f84bcd7e3399c88989305ea565497c030d9

src/com/android/launcher3/BubbleTextView.java

@@ -505,7 +505,7 @@ public class BubbleTextView extends TextView implements ItemInfoUpdateReceiver,
      * @param canvas The canvas to draw to.
      */
     protected void drawDotIfNecessary(Canvas canvas) {
-        if (mDisplay == DISPLAY_TASKBAR) {
+        if (mActivity instanceof Launcher && ((Launcher) mActivity).isViewInTaskbar(this)) {
             // TODO: support notification dots in Taskbar
             return;
         }