Rework navbar support

Most devices have hardware buttons but some do not, and need a navbar.

Support this in a generic and consistent way:

* Set com.android.systemui.statusbar.dpi=0 for default configs.

* Introduce pa_nav_*.conf for devices that want navbar by default.

* Set mako to use navbar by default.

Device maintainers, please switch your devices to use pa_nav_*.conf as
needed.  If there are any hdmi/mdpi/ldpi devices that want/need a navbar
by default, copy the generic config to pa_nav_*.conf and set the navbar
config appropriately.

Change-Id: Ief2306c42ecc407e9d0e091076c3755560d85c44