Runtime toggle of navbar

This adds the framework support for enabling the Navigation bar
on devices with hardware keys. It is toggled from Settings, and
depends on device-specific support for the KeyDisabler hardware
control

Change-Id: I88fecb2ca1e8613591c327a93f53909b00239cd8

wm: Nullify hardkey function assignments if enabling the navbar

This caused erroneous (and sometimes duplicate) events being generated
due to the regular key function assignments. The navbar does its
own action management, so don't try to derive from the actions
usually present in hard keys.

Change-Id: I82866e24547f8145cac4f07820ae90aacce09281