Clean up code warnings

lib/ui/auth/lock_screen.dart

@@ -38,7 +38,7 @@ class LockScreen extends StatelessWidget {
               ),
             ],
           ),
-          RaisedButton(
+          ElevatedButton(
             onPressed: onAuthenticatePressed,
             child: Text(localization.authenticate),
           )

lib/ui/settings/user_details.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 import 'package:flutter_styled_toast/flutter_styled_toast.dart';
 import 'package:invoiceninja_flutter/redux/app/app_actions.dart';
 import 'package:invoiceninja_flutter/ui/app/forms/app_dropdown_button.dart';
@@ -572,7 +571,7 @@ class _EnableTwoFactorState extends State<_EnableTwoFactor> {
                         ),
                       ],
                     ),
-                    if (false)
+                    /*
                     Row(
                       children: [
                         Expanded(
@@ -595,6 +594,7 @@ class _EnableTwoFactorState extends State<_EnableTwoFactor> {
                         ),
                       ],
                     ),
+                    */
                   ],
                 ),
               ),

samples/form_keys.dart

@@ -222,9 +222,7 @@ class ContactsPageState extends State<ContactsPage>
 
     items.add(Padding(
       padding: const EdgeInsets.all(16),
-      child: RaisedButton(
-        elevation: 4.0,
-        textColor: Theme.of(context).secondaryHeaderColor,
+      child: ElevatedButton(
         child: Text('ADD CONTACT'),
         onPressed: _onAddPressed,
       ),

samples/form_redux.dart

@@ -257,9 +257,7 @@ class ContactsPage extends StatelessWidget {
           ..addAll(contacts)
           ..add(Padding(
             padding: const EdgeInsets.all(16),
-            child: RaisedButton(
-              elevation: 4.0,
-              textColor: Theme.of(context).secondaryHeaderColor,
+            child: ElevatedButton(
               child: Text('ADD CONTACT'),
               onPressed: () {
                 store.dispatch(AddContact());