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