Replace state$ with atRest$

[appsforartists]

We changed from atRest$ to state$ because some members of the core team had strong feelings about it being enumerated (I think they found atRest:true to be strange to reason about). We should revisit this decision:

• We have aggregators now that work on true/false. If state$ is emitting enums, we have to manually map state$ to true/false or can't use the aggregators. For instance, see the complexity in aggregating tossable.state$
• Interactions often interact with state machines: for instance, whether a bottom sheet is open or closed. state$ makes more sense as a name for interaction-specific state than for activity.