Settings: Replace CheckboxPreference with BoolPreference

It's better to have our own small class for this as the one provided
with the library tries to contact the PreferencesService and that isn't
what we want.

lib/screens/settings_screen.dart

@@ -166,30 +166,22 @@ class SettingsListState extends State<SettingsList> {
       ),
       SizedBox(height: 16.0),
       SettingsHeader("Analytics"),
-      CheckboxPreference(
-        "Collect Anonymous Usage Statistics",
-        "usage_stats",
-        defaultVal: Settings.instance.collectUsageStatistics,
-        onEnable: () {
-          Settings.instance.collectUsageStatistics = true;
-          Settings.instance.save();
-        },
-        onDisable: () {
-          Settings.instance.collectUsageStatistics = false;
+      BoolPreference(
+        title: "Collect Anonymous Usage Statistics",
+        defaultValue: Settings.instance.collectUsageStatistics,
+        onChange: (bool val) {
+          Settings.instance.collectUsageStatistics = val;
           Settings.instance.save();
+          setState(() {});
         },
       ),
-      CheckboxPreference(
-        "Collect Anonymous Crash Reports",
-        "crash_reports",
-        defaultVal: Settings.instance.collectCrashReports,
-        onEnable: () {
-          Settings.instance.collectCrashReports = true;
-          Settings.instance.save();
-        },
-        onDisable: () {
-          Settings.instance.collectCrashReports = false;
+      BoolPreference(
+        title: "Collect Anonymous Crash Reports",
+        defaultValue: Settings.instance.collectUsageStatistics,
+        onChange: (bool val) {
+          Settings.instance.collectCrashReports = val;
           Settings.instance.save();
+          setState(() {});
         },
       ),
       VersionNumberTile(),
@@ -350,3 +342,27 @@ class ListPreference extends StatelessWidget {
     );
   }
 }
+
+class BoolPreference extends StatelessWidget {
+  final String title;
+  final bool defaultValue;
+  final Function(bool) onChange;
+
+  BoolPreference({
+    @required this.title,
+    @required this.defaultValue,
+    @required this.onChange,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return ListTile(
+      title: Text(title),
+      trailing: Checkbox(
+        value: defaultValue,
+        onChanged: onChange,
+      ),
+      onTap: () => onChange(!defaultValue),
+    );
+  }
+}