Disallow action btn to be clicked multiple times or when dismissing

Previously, the action button could be clicked multiple times (spam click while dismissing) which would result in the onActionClicked() callback being called multiple times. Considering the common uses of Snackbar, I feel like that should be undesired. Now, when the action button is clicked, it checks two things before calling the callback:

1) The snackbar is not dismissing
2) That it is the first time the button is being clicked unless multiple clicks are allowed (disabled by default)