Preferences fix

[lorenzoPrimi]

Fixes: ooni/probe#1024

The "hack" I was using to add a fake <Preference /> in the global preferences and replacing it at runtime with the "real" test settings with this code

if (getString(R.string.websites).equals(rootKey))
            setPreferencesFromResource(R.xml.preferences_websites, getArguments().getInt(ARG_CONTAINER_RES_ID), rootKey);

Caused ooni/probe#1018
so I removed this code and went for a clean approach with only one xml file for preferences.

I tried to split one single preference screen in multiple files without any luck, at the moment I would classify it as L/XL and we have other priorities.

[bassosimone]

🐳