Fix power widget appearing after theme change even if disabled.

The power widget used a 'boot completed' listener to initially configure
its appearance. As the status bar is restarted when themes are changed,
and obviously no boot completed intent followed that, the power widget
stayed visible, even if it's disabled in the settings.
Fix that by reading the initial settings in the constructor instead of a
boot completed receiver. I couldn't find any negative impact after that
change, so the exact purpose of the original code (using a boot
completed receiver) is unclear.