Support in `carbon-route` for array values in `queryParams`

[cdata]

As a custom element author, when I extend carbon-route and friends, I want to assume that queryParams will gracefully handle array properties, so that I do not need to implement support myself.

Currently, we observe for sub-property changes on queryParams in carbon-route, but we do not actually support handling splices. In fact, splice changes will result in undefined behavior. Right now, this isn't a problem because we do not deserialize arrays in iron-query-params. However, we should support splices in the long-run so that we are not locked into iron-query-params implementation for this kind of thing.