Don't restore old brightness when cancelling brightness dialog in automatic mode.

Fixes a problem that could result in the backlight getting set to a bogus brightness
level if you open the dialog and cancel when in automatic mode.

Bug b/2015734

Change-Id: I302697e4ebd37afcdbfdd74ba0b1cc9be27ee40b
Signed-off-by: Mike Lockwood <lockwood@android.com>