@Override
    public boolean onPreferenceChange(Preference preference, Object value) {
        setPreferenceSummary(preference, value);
        return true;
    }
Attaches a listener so the summary is always updated with the preference value.
Also fires the listener once, to initialize the summary (so it shows up before the value
is changed.)
app/src/main/java/com/example/android/sunshine/app/SettingsActivity.java