Remove member variable initialization

This is being injected by the instrumentationTests for mocking. So use that instead of creating an actual one.

collect_app/src/main/java/org/odk/collect/android/widgets/ExStringWidget.java

@@ -107,7 +107,7 @@ public class ExStringWidget extends QuestionWidget implements BinaryWidget {
     private final Button launchIntentButton;
     private final Drawable textBackground;
 
-    private ActivityAvailability activityAvailability = new ActivityAvailability(getContext());
+    private ActivityAvailability activityAvailability;
 
     public ExStringWidget(Context context, FormEntryPrompt prompt) {