toast: needs to support non-visual & keyboard users

[marcysutton]

To be accessible, material-toast needs to be exposed to users who cannot see it appear. It must also be operable without a mouse and be discoverable with touch. There are a few things involved:

1. Toast contents can be read aloud in an ARIA live region (recommended setting: "polite"). Region must be bootstrapped on page load and be part of the page's render flow (meaning it cannot be hidden with display: none). Offscreen CSS works fine for this.
2. The escape key should operate in a "stack" to close actions in the order they were triggered.
3. Any actions inside of the Toast need to be discoverable and operable from the keyboard or touch (Undo, etc.).

Escape stack tool idea from @ajoslin: https://gist.github.com/ajoslin/498892d942e4fc5c552c