From the Processing Android Wiki, within the keyPressed function, you call:

if(key == CODED && keyCode == KeyEvent.KEYCODE_BACK) {
       keyCode = 0;
       //Do stuff here
} 

1. 
   import android.view.KeyEvent;
   
   int state = 0;
    
   
   void draw() {
     if (state == 0)
       background(255, 0, 0);
     else if (state == 1)
       background(0, 0, 255);
   }
   
   boolean surfaceKeyDown(int code, KeyEvent event) {
     if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
       if (state == 0) {
         state = 1;
       }
       else {
         state=0;
       }
     }
     return false;
     //return true actually also works...
    
   }
   
   boolean surfaceKeyUp(int code, KeyEvent event) {
     return true;
     //return false does not work here
   }