Fix file picker compatibility with older Android versions for json import

See (historic) SO issue:
https://stackoverflow.com/a/58104204
and introduction of the json mime type in 2018:
https://android.googlesource.com/platform/libcore/+/d5e73ac1a2c92dd929e1462b63d6e36509813486

Author: ray2c

lib/services/backup/backup_service.dart

@@ -36,7 +36,9 @@ class ManualBackupService {
 
   /// Imports all the notes from a JSON file picked by the user and returns whether the import was successful.
   Future<bool> import(BuildContext context) async {
-    final importedFile = await selectAndReadFile(MimeType.json.value);
+    // Compatibility: the json mime type was only introduced in Android 10; just specify binary for older versions
+    final mimeType = InfoUtils().androidVersion < 29 ? "application/octet-stream" : MimeType.json.value;
+    final importedFile = await selectAndReadFile(mimeType);
 
     if (importedFile == null) {
       return false;