We know Sony SmartEyeglass has a host app installed on Android device and a wired controller. It seems that the HostApp is a service without UI running on Android. Is it possible to build an Android app with user interface to control the glass?
SmartEyeglass extension is an Android app. If you start developing from sample projects available in SmartEyeglass SDK, you will find that none of them has a Launcher Activity. But you can add any extra activity. That can also provide extra controls yo your SmartEyeglass app.
Note: HostApp has also a UI, which you can access by the SmartEyeglass task in system tray, when SmartEyeglass device or emulator is connected. It is used mostly for configuration the device and his applications. Host app is responsible for communication between your extension and the host device.