[Power] Fix Power API failures on tct test suites.

-Remove workaround of glib callback as it is not needed anymore : the
extensions run in a separate process.
-Remove commented code in the examples as now switching on and off doesn't
background crosswalk behind the launcher.
[Power] Make test PowerManager_setScreenBrightness_brightness_TypeMismatch pass.

We need to throw the right error when the brightness passed is -Infinite,
+Infinite or NaN.
Merge pull request #143 from darktears/power-fixes

Power fixes