Factorize positon computation + Removed ViewTreeObserver

[darzul]

There were two places in the code to compute the scrolling position. As it is the same code I factorized it in a method -> setBubbleAndHandlePosition()

In this PR the ViewTreeObserver on RecyclerView is replaced by a call to setBubbleAndHandlePosition() in onSizeChanged(). When the RecyclerViewFastScroller size changes we have to compute the new handle position.

What do you think about these changes ?

[AndroidDeveloperLB]

Nice. So it's only refactoring, right?
I'd name the new function differently thought. Maybe "updateBubbleAndHandlePosition".
When having a "set", this prefix is usually for setters.

[darzul]

Yes it is.
You're right, it's fixed :)

[AndroidDeveloperLB]

ok then. thanks for helping.