rework the onDraw method

[mrwonderman]

Try and remove the instantiating of various objects in onDraw(). There are these 3 cases:

• getFolderPath()
• getDividerPath()
• getWritableFolderRect(int cw)

As suggested by: https://plus.google.com/u/0/+YannickSigner/posts/iT8DwRXaE1P?cfem=1

You can reuse Rect objects instead of creating new ones. As for the Path, you should only rebuild it when it changes (like when the bounds of the view change for example) then reuse it.