Changed EventListener methods to take a Snackbar instance as the paramet...

...er.

This allows users to:
- differentiate between Snackbar instances in common EventListener
  implementations
- get the width, height, and offset from the Snackbar in pixels (or any other
  relevant info about the Snackbar)

Also made sure that the width/height values are valid when onShow() is called
and prevented the dismiss listener from making the height of the Snackbar 0
before calling onDismiss() when swiping the Snackbar away (this logic was there
for swiping away list items, which a Snackbar is not).