Slight update on a old, but good trick.
* Sometimes, you might have to add a delay between the intent Action and the Input Action
* On phones that don't have a Camera button (Samsung Galaxy S5), sometimes the Camera button Action doesn't do anything. Instead, Use a "Input>Dpad" Action, and for the "Button", use "Press" and have it repeat twice. The first press will select the videocamera button, and the second will start recording.