Fix ScheduledActionService.autoBackup() not doing backups of all books

It was doing a backup of just the active book.

It solves the problem only for the case in which the backup is created
in the default diretory. Pending to solve the case in which the user
specifies a target file.

app/src/main/java/org/gnucash/android/service/ScheduledActionService.java

@@ -274,7 +274,7 @@ private static void autoBackup(){
         for (String bookUID : bookUIDs) {
             String backupFile = BookUtils.getBookBackupFileUri(bookUID);
             if (backupFile == null){
-                GncXmlExporter.createBackup();
+                GncXmlExporter.createBackup(bookUID);
                 continue;
             }