fixed enum serialization

lib/model/alarm/fixed_time_alarm.dart

@@ -24,7 +24,7 @@ class FixedTimeAlarm implements Alarm {
     return {
       'id': id,
       'triggerAt': triggerAt.toIso8601String(),
-      'alarmType': alarmType,
+      'alarmType': alarmType.toJson(),
     };
   }
 }

lib/model/alarm/location_alarm.dart

@@ -32,7 +32,7 @@ class LocationAlarm implements Alarm {
       'id': id,
       'location': location.toJson(),
       'radiusMeters': radiusMeters,
-      'alarmType': alarmType,
+      'alarmType': alarmType.toJson(),
     };
   }
 }

lib/model/alarm/relative_time_alarm.dart

@@ -24,7 +24,7 @@ class RelativeTimeAlarm implements Alarm {
     return {
       'id': id,
       'triggerAt': triggerAt.toIso8601String(),
-      'alarmType': alarmType,
+      'alarmType': alarmType.toJson(),
     };
   }
 }