Fix http://b/2305223

Fix http://b/2305223

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).