chromecast: fix detection of the homescreen

As reported by @nog3, the home screen ID has changed on recent
Chromecasts. We try to accommodate not-yet-upgraded Chromecasts and
up-to-date Chromecasts by allowing any of the ID to be detected as the
homescreen.

Should fix #10.
chromecast: display a debug message when checking for home screen

Otherwise, if the Chromecast is not on home screen, the user may not see
anything.