Use ISO 8601 date formatting

Update TasksJsonExporter and the default toString() method in DateTime to use the ISO 8601 standard (in particular, using four-digit years and splitting the date from the time with the character "T").

app/src/main/java/org/tasks/backup/TasksJsonExporter.kt

@@ -169,6 +169,6 @@ class TasksJsonExporter @Inject constructor(
         private const val MIME = "application/json"
         private const val EXTENSION = ".json"
         private val dateForExport: String
-            get() = newDateTime().toString("yyMMdd-HHmm")
+            get() = newDateTime().toString("yyyyMMdd'T'HHmm")
     }
 }

app/src/main/java/org/tasks/time/DateTime.java

@@ -429,6 +429,6 @@ public class DateTime {
 
   @Override
   public String toString() {
-    return toString("yyyy-MM-dd HH:mm:ss.SSSZ");
+    return toString("yyyy-MM-dd'T'HH:mm:ss.SSSZ");
   }
 }