bugid: 94988 -- refactor ShareButton class to fix fatal on preview

SharingToolbarController was constructing ShareButton instances to access what could have been a static property on each child. Because we want instantiated logic to be dependent upon a Title instance, and since the getAssets call should be relatively stateless, I have refactored the class hierarchy a little bit. At the same time, I've removed the assumption that a couple of generally unused rendering functions must be implemented by their children, since this doesn't make a great deal of OO sense. 

This fix addresses a P2 defect on preview.