fixed an issue with null schedule mode for older notifications scheduled using periodicallyShow on Android (#2076)

Author: MaikuB

flutter_local_notifications/CHANGELOG.md

@@ -1,5 +1,6 @@
 # [16.0.0-dev.2]
 
+* [Android] fixed issue an issue similar to [2033](https://github.com/MaikuB/flutter_local_notifications/issues/2033) that was addressed in 15.0.1 where notifications on scheduled using older version of the plugin via the `periodicallyShow()` method would fail to have the next subsequent ones scheduled. This issue started occuring in 14.0 where support for inexact notifications was added using the `ScheduleMode` enum that was added and resulted in the deprecation of `androidAllowWhileIdle`. A mechanism was added to help "migrate" old notifications that had `androidAllowWhileIdle` specified but didn't account for how there are recurring notifications that were scheduled using older versions of the plugin prior to `androidAllowWhile` being added
 * Updated example app so that the Android side specifies minimum SDK version version that aligns with what's specified by the Flutter SDK
 * Fixed Dart API docs for `DarwinNotificationDetails` class where `this This` was being repeated. Thanks to the PR from [Adrian Jagielak](https://github.com/adrianjagielak)
 

flutter_local_notifications/android/src/main/java/com/dexterous/flutterlocalnotifications/FlutterLocalNotificationsPlugin.java

@@ -673,6 +673,14 @@ private static void repeatNotification(
         getBroadcastPendingIntent(context, notificationDetails.id, notificationIntent);
     AlarmManager alarmManager = getAlarmManager(context);
 
+    if (notificationDetails.scheduleMode == null) {
+      // This is to account for notifications created in older versions prior to allowWhileIdle
+      // being added so the deserialiser.
+      // Reference to old behaviour:
+      // https://github.com/MaikuB/flutter_local_notifications/blob/4b723e750d1371206520b10a122a444c4bba7475/flutter_local_notifications/android/src/main/java/com/dexterous/flutterlocalnotifications/FlutterLocalNotificationsPlugin.java#L642
+      notificationDetails.scheduleMode = ScheduleMode.inexact;
+    }
+
     if (notificationDetails.scheduleMode.useAllowWhileIdle()) {
       setupAllowWhileIdleAlarm(
           notificationDetails, alarmManager, notificationTriggerTime, pendingIntent);
@@ -1613,7 +1621,7 @@ private void getCallbackHandle(Result result) {
     result.success(handle);
   }
 
-  /// Extracts the details of the notifications passed from the Flutter side and also validates that
+  // Extracts the details of the notifications passed from the Flutter side and also validates that
   // some of the details (especially resources) passed are valid
   private NotificationDetails extractNotificationDetails(
       Result result, Map<String, Object> arguments) {