don't play target tokens on practice activity init if it's a hidden word activity (#1121)

lib/pangea/models/practice_activities.dart/practice_activity_model.dart

@@ -222,6 +222,10 @@ class PracticeActivityModel {
 
   String get question => content.question;
 
+  bool get shouldPlayTargetTokens =>
+      targetTokens != null &&
+      activityType != ActivityTypeEnum.hiddenWordListening;
+
   factory PracticeActivityModel.fromJson(Map<String, dynamic> json) {
     // moving from multiple_choice to content as the key
     // this is to make the model more generic

lib/pangea/widgets/practice_activity/multiple_choice_activity.dart

@@ -63,7 +63,8 @@ class MultipleChoiceActivityState extends State<MultipleChoiceActivity> {
   }
 
   void speakTargetTokens() {
-    if (widget.practiceCardController.currentActivity?.targetTokens != null) {
+    if (widget.practiceCardController.currentActivity?.shouldPlayTargetTokens ??
+        false) {
       widget.practiceCardController.tts.tryToSpeak(
         PangeaToken.reconstructText(
           widget.practiceCardController.currentActivity!.targetTokens!,