Modified calls to onShow(height) and onDismiss(height) so they both occu...

...r when the action is starting and they both provide an exact height in pixels instead of a maximum height in dp. This helps in the implementation of floating action buttons movement with respect to the snackbars as specified in the material guidelines.

Also removed some unneeded calls to dpToPx().
Moved here from show() since getHeight() does not yet returns a proper value there and the ViewTreeObserver.addOnDrawListener() requires api level 16.