Snackbar: Designer feedback

[acdvorak]

Design has requested the following changes to the new Snackbar (PR #4091):

• 100% width on "mobile-sized" screens
• Remove mdc-snackbar--wide class and add note to README about how to achieve wide margins on larger devices using @media queries
• Remove Sass variables for animation delays
  (The motion spec has a 150ms delay for exit animations, but according to Design, that's a mistake; they should both always be 0ms)
  • $mdc-snackbar-enter-delay
  • $mdc-snackbar-exit-delay