Add hands-free voice actions to Dex Mobile

Add a hands-free voice mode to Dex Mobile so users can work with Dex while driving or otherwise unable to type.

Desired outcome: let users speak and review supported actions such as looking up contact context, adding notes and reminders, sending messages with attachments, creating calendar events, and starting calls. Before any external action, Dex should show what it understood and require the appropriate confirmation.

Third-party purchases or other transactional actions need separate product and safety review.