livedisplay: Add an "off" state

 * Don't force "day" to be the default state.
 * Change minimum value for color calibration to 20% so blackout doesn't
   happen when all sliders are at minimum.
 * Also improve a few strings to describe what day and night means.

Change-Id: Ib2a617488fffb128c8e3e9e52d64fac6b261e53d