Settings: correct y-axis brightness in preview

To answer the comment in the code, no, the min brightness should not
correspond 0, but rather to the device min supported level. The values
on the preview graph should correspond exactly to the percent values
entered in the brightness configuration dialog.

Change-Id: I4a6d19b261f33f53d24437cde99e63e03ba3fc28
Merge "Settings: correct y-axis brightness in preview" into cm-11.0