Use persistent service to attach activity over clock screen

[ysamlan]

Maybe in addition or instead of the separate activity UI. This might let us intercept touches even afer the screen was turned off with the palm-over-screen gesture, and maybe let the user see their default watchface (Baby Time could just be a notification + service instead of a separate app?)

We'll probably have to attach a View directly to the windowmanager from the service, and be very careful that when deactivated we stay out of the way and kill the service.