Allow different visual and voice assist prompts

* Introduce VoiceInteractor.Prompt, holding multiple voice prompts
("What genre?", "What genre of music?", "What genre of music, for
example classical?") as well as a different visual prompt to show
on screen ("Choose genre").
* Migrate framework voice interactor code from a CharSequence prompt
to Prompt.

Bug: 21024958
Change-Id: Ib595fbdb2801cc558085e9b8366d619ff1d4d656