Hide "Magnify with button" if no navbar is shown

Devices without a software-rendered navigation bar will be unable to
use "Magnify with button" as no accessibility button trigger for
magnification can be shown.

In this case, we hide the setting by preventing
MagnificationPreferenceFragment from showing the list of mag
modes, instead launching directly into the PreferenceFragment for
"Magnify with triple-tap"

Bug: 36862906
Test: Manual - Configured marlin to disable the navigation bar and
      verified behavior in SUW and Settings

Change-Id: Ie614ec292d7779044274b7c79eaed1df080c0fd1