Add support for runtime toggle of navbar

This is a refactor of the following changes, so to make use of tuner API.

Author: Timo Wendt <timo@tjwendt.de>
Date:   Thu Aug 30 12:18:41 2012 +0300

    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

    Update DEV_FORCE_SHOW_NAVBAR constant.

    Change-Id: Ie5b4317162c514d22276956f81007e064a3d0f32

    Settings: Move DEV_FORCE_SHOW_NAVBAR load to loadSecureSettings.

    Change-Id: I6ac53b8c9f7fce6f9ca6b4ad7bf31a1c1e896863

Author: Paul Keith <javelinanddart@gmail.com>
Date:   Tue Jan 16 15:47:07 2018 +0100

    PhoneWindowManager: Make sure KeyDisabler is always called on boot

    * Otherwise, some KeyDisabler classes are left in a weird state
    * Because we don't keep track of whether an initial state was ever set,
      we never call KeyDisabler on boot if the setting is set to 0
    * To remedy this, keep track of whether an initial state was set

    Change-Id: Ib432ed3278dd8f4f4cba3ba488879b3c1cd9c8f4

Change-Id: I4a6d3f89bc171c3921875b24c077cb78c03517ad