Create a "single-top" MenuHost

[boswelja]

We now have two default implementations of MenuHost!

• The original implementation has been renamed to CumulativeMenuHost. As its name suggests, it accumulates all provided MenuItems and exposes them all at once. This is useful for breaking down bigger menus into separate Composables.
• A new SingleTopMenuHost! This MenuHost will collect all sets of MenuItems, and expose only the last set of items. This should be useful for screens that provide toolbar items, since you don't (typically) want to mix screen actions.