converted folding circles progress bar to folding circles drawable

So here is quite a big pull request, not sure if you agree with this one.

Here is the fixes it brings:
- converted `FoldingCirclesProgressBar` to `FoldingCircleDrawable` so it can be reused in many other situations. plus it makes the use of it more clean from a progress bar (that's for #2)
- cleaned up the code a bit. the new class is a bit lighter than the first one
- the animation speed can be modified easily now with `android:indeterminateDuration` on the `ProgressBar` (that's for #1)