Add two properties to KotlinInvadersView to represent and control the numbers of the invaders

KotlinInvadersView.kt

...
...
// The players ship
private var playerShip: PlayerShip = PlayerShip(context, size.x, size.y)

 // Some Invaders
 private val invaders = ArrayList<Invader>()
 private var numInvaders = 0

 // The score
 private var score = 0
...
...