EVA FACIAL MOUSE is a free and open source application developed with funding from Vodafone in Spain. It uses the webcam to track the movement of your head, it allows you to control a pointer on the screen (as a mouse) simply stop moving and it activates the dwell click (settings can be changed in the menu)
Also available for Windows computer working in the same way via your webcam
This is a great app that works well with a little bit of practice and it's free !
The app is available for free from the Play Store
Very comprehensive setup guide is available here or in the videos below. The original video is in Spanish but English subtitles are available in the video menu.