Fix http://b/2305223 (DO NOT MERGE)

When placed into night mode (dim mode), the DeskClock must
restore full brightness any time the user returns to it from
another activity. There are two reasons for this:

1. The other activity won't be dim, so it's sort of jarring
   to return to an almost-black screen.

2. As indicated in bug 2305223, visiting another activity is
   a useful way to restore the display if it's so dark that
   you can't see it to find the nightmode toggle button
   (e.g., ambient lighting is very bright).