Add hasNavigationBar() to the window manager.

It is no longer sufficient to check the value of
internal.R.bool.config_showNavigationBar to determine if a
navigation bar (separate from the status bar) is shown on a
device, because the emulator needs to be able to override
this value (now possible by setting qemu.hw.mainkeys to "1"
or "0", for navbar or no navbar, respectively).

This logic is now contained in PhoneWindowManager, and any
clients wishing to know whether the system has a software
nav bar should consult the new hasNavigationBar() method.

Bug: 5404945
Change-Id: I119d32a8c84b88b2ef46f63244e7f11dc5de0359