Closes #1172: Make RadioButton in RadioButtonPreference non focusable.

This tells TalkBack not to land on the button individually, but instead
to land on the parent View. TalkBack users will get both the button
state and the textual description. Double tapping should work too.

app/src/main/res/layout/preference_widget_radiobutton.xml

@@ -28,6 +28,7 @@
                 android:layout_gravity="start"
                 android:button="@null"
                 android:clickable="false"
+                android:focusable="false"
                 android:drawableStart="?android:attr/listChoiceIndicatorSingle"
                 android:drawablePadding="@dimen/radio_button_preference_drawable_padding"/>