Fix a potential crash caused by ConcurrentModificationException

[YkSix]

Crash logs

java.util.ConcurrentModificationException
  at java.util.ArrayList$Itr.checkForComodification
  at java.util.ArrayList$Itr.next
  at com.linecorp.apng.ApngDrawable.progressAnimationElapsedTime
  at com.linecorp.apng.ApngDrawable.draw
  at android.widget.ImageView.onDraw

Steps to reproduce

If the client registers an animation callback multiple times—potentially due to timing issues or intentional actions by client apps—and then unregisters themselves during the callback execution (a reasonable approach), it triggers a ConcurrentModificationException.

Sample code

// repeat this action multiple times.
apngDrawable.registerAnimationCallback(object : Animatable2Compat.AnimationCallback() {
    override fun onAnimationEnd(drawable: Drawable?) {
        apngDrawable.unregisterAnimationCallback(this)
    }
})

[CLAassistant]

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[r-ralph]

LGTM!
Thank you for wrinting test code too :)