Hide subtasks option on pre-marshmallow devices

app/src/main/java/org/tasks/preferences/BasicPreferences.java

@@ -2,6 +2,7 @@ package org.tasks.preferences;
 
 import static com.todoroo.andlib.utility.AndroidUtilities.atLeastJellybeanMR1;
 import static com.todoroo.andlib.utility.AndroidUtilities.atLeastLollipop;
+import static com.todoroo.andlib.utility.AndroidUtilities.atLeastMarshmallow;
 import static java.util.Arrays.asList;
 import static org.tasks.PermissionUtil.verifyPermissions;
 import static org.tasks.dialogs.ExportTasksDialog.newExportTasksDialog;
@@ -322,6 +323,8 @@ public class BasicPreferences extends InjectingPreferenceActivity
         R.string.p_language,
         R.string.p_layout_direction);
 
+    requires(R.string.task_list_options, atLeastMarshmallow(), R.string.p_show_subtasks);
+
     requires(BuildConfig.DEBUG, R.string.debug);
 
     //noinspection ConstantConditions

app/src/main/res/xml/preferences.xml

@@ -22,7 +22,9 @@
 
   </PreferenceCategory>
 
-  <PreferenceCategory android:title="@string/task_list_options">
+  <PreferenceCategory
+    android:key="@string/task_list_options"
+    android:title="@string/task_list_options">
 
   <com.todoroo.astrid.ui.MultilinePreference
       android:key="@string/p_default_list"