Instant Search: aria-live accessibility improvements

This is an issue to track accessibility improvements we'd like to make to the Jetpack Instant Search prototype:

- [x] add `aria-live` to search results (https://github.com/Automattic/jetpack/issues/13644)
- [x] add a label to the search box (even if hidden from most users) (https://github.com/Automattic/jetpack/pull/13875)
- [x] add a button to the search (even if hidden from most users) (https://github.com/Automattic/jetpack/pull/13875)
- [x] add a `` tag with `role="search"`
- [ ] [`aria-live` + VoiceOver + Safari](https://github.com/Automattic/jetpack/pull/13884#issuecomment-549970221): results announcement is interrupted by the announcement of text field content
- [ ] [`aria-live` + NVDA + Firefox](https://github.com/Automattic/jetpack/pull/13884#issuecomment-549978656): No announcements made for search results
- [ ] [`aria-live` + NVDA + Chrome](https://github.com/Automattic/jetpack/pull/13884#issuecomment-549978656): announces from the start of some article at the end of the results and then goes to the previous article. Does not announce `n results for y`.

Start with the unchecked VoiceOver/Safari and NVDA/Firefox/Chrome cases described in this issue and inspect the Instant Search prototype and the referenced pull request discussion. Reproduce each browser and screen-reader combination, then verify that search-result announcements are not interrupted, are emitted consistently, and include the result count and query.

Written by the indexing model from the issue text.