Low power mode display is disabled by default

Set use_low_power_mode=false both in gconf schema and builtin gconf.

Skip schema installation if mce is configured to use builtin-gconf.

The builtin-gconf stores persistently only keys that have a value that
differs from the default. This is needed so that future configuration
changes take effect without resetting the value cache file.

Also routed all high brightness mode changes via helper function that
clips values to valid range before writing to sysfs (and does diagnostic
logging for out of bounds values).