Hideybars feature confirmation toast bar.

Enhance Toast to support new standard toast bar style, similar
to the undo bar in Gmail.  Toast bars can be interactive,
and can have a single action.  Add a new toast duration to
indicate persistent toasts (no auto-hide delay).

Use the new toast bar to implement a feature hint when hiding
the navigation bar in hideybars mode.

Per UX, the feature confirmation bar can also be dismissed on
any outside touch as long as the user confirmed it using the
OK button at least once globally.

Bug: 8754108
Change-Id: Iaa85d3b4da7ada1952a562f1e31de04380f5d587