Consecutive snackbars Specs is mismatched from Google Material Specs

[zskhan]

There is only ever one snackbar displayed. When a second snackbar is triggered while the first is displayed, the first should start the contraction motion downwards before the second one animates upwards.
https://material.google.com/components/snackbars-toasts.html#snackbars-toasts-usage

[zskhan]

#286
Here is my proposed fix

[mseemann]

fixed