power: Switch to enum classes.

Update powerd's enums to use "enum class". This was almost
entirely mindless, but I also:

- refactored WakeupController's IsUsableInMode() helper
  function to use a switch statement instead of a bunch of
  chained if statements
- removed the default cases from common/power_constant.cc's
  ToString functions and KeyboardBacklightController's
  GetTransitionDuration function
- renamed BacklightController's TransitionStyle enum to
  Transition
- added some <memory> and <utility> includes that the
  presubmit script nagged me about

BUG=none
TEST=tests pass; also deployed to a device and confirmed
     that it appears to be functioning the same as before
     (i.e. suspend/resume works)

Change-Id: Id7be7a008104f0c838202151e94dafdf707779b8
Reviewed-on: https://chromium-review.googlesource.com/408653
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>
60 files changed