[Settings] Nav bar tweaks

- Toggle on/off (CyanogenMod alredy did but only for hw keys devices). crDroid allow all devices to do that for those using PIE apps.
- Nav bar dimensions (height and width)

PS: THIS COMMIT IS A SQUASHED PORTED FROM crDroid KK and 99% od the codes are from SlimRoms. I want to say a HUGE thanks to @kufikugel