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
Populates and displays a warning message when a feature that relies
on the Accessibility button is enabled, but not assigned.
Bug: 34720082
Test: Manual
Change-Id: Ie7031716a91fdf7fc40f32bf4e0ad17af85aa573
