The reason for the junk values is due to,
this._oDialog.open(this);
If you want no values in the search field, it has to be,
this._oDialog.open();
open method takes an argument which will be set as the value for the search field. In your case as you're passing *this* as an argument, the Object is converted into String and set as the value for search field.
JsDoc Report - SAP UI development Toolkit for HTML5 - API Reference - sap.m.SelectDialog