Bug 1093951 - Entering wifi power save mode when screen off. r=alive

While screen is off, wifi would go to power save mode only
when wifi lock is held. If wifi lock is not held, wifi would
stay on and be turned off after |wifi.screen_off_timeout|.
We can more aggressively request the wifi driver to enter
power save mode before timed out. This is what the patch
intends to do.