Some post-merge cleanup for automatic brightness adjustment

- Remove some unused strings
- Use (device-dependant) minimum brightness level (as calculated by
  DisplayPowerController) as '0%' instead of the actual value 0.

Change-Id: I79123d2421c9ab35fa0f1808d82d785463e8fbaa
Merge remote-tracking branch 'upstream/cm-10.1' into cm-10.1