Separate configuration of auto-brightness for button/keyboard backlights.

Presently, light-sensor based automatic brightness
(config_autoBrightnessLevels) controls both button and keyboard backlights
in addition to the screen backlight.  Unfortunately auto-brightness for
button/keyboard backlights doesn't work well on devices (e.g., epicmtd)
with neither (i) dimmable button/keyboard backlights (so when they're on,
they're very bright even if the screen backlight is relatively dark) nor
(ii) identifiable surface features on capacitive buttons (so they can't be
found if the backlights are permanently turned off).

For these devices, the non-auto-brightness behavior of button/keyboard
backlights, whereby they turn on during screen-on events and user activity
and timeout-to-off shortly thereafter, is preferable.  Thus, this commit
adds a config option, config_autoBrightnessButtonKeyboard (defaults to
true), that when disabled allows for automatic brightness control of the
screen backlight, but retains the timeout-based behavior for button and
keyboard backlights.

Also addressed is a bug that disables the button and keyboard backlights on
screen unlock, and reenables the backlights on touch events so that
capacitive keys may be located.