Fatal Exception: java.util.ConcurrentModificationException

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Issue Info

Info	Value
Device Info	Xiaomi Redmi Note9, Samsung Galaxy A7, etc..
System Version	8.0-14.0
APNG4Android Library Version	2.28.0
Repro rate	sometimes
Repro with our demo project	-
Demo project link	-

Issue Description and Steps

We get the following exception in our project from time to time. It seems to happen when stop and unregisterAnimationCallback are called close to one another.

My guess is that https://github.com/penfeizhou/APNG4Android/blob/master/frameanimation/src/main/java/com/github/penfeizhou/animation/FrameAnimationDrawable.java#L60 is still not safe since copying over a set to an array most likely involves an iteration. Which can cause an exception if the set is modified at the same time.

I can try to write a demo project which reproduces the exception if necessary.

       Fatal Exception: java.util.ConcurrentModificationException:
       at java.util.HashMap$HashIterator.nextNode(HashMap.java:1441)
       at java.util.HashMap$KeyIterator.next(HashMap.java:1465)
       at java.util.AbstractCollection.finishToArray(AbstractCollection.java:232)
       at java.util.AbstractCollection.toArray(AbstractCollection.java:143)
       at java.util.ArrayList.<init>(ArrayList.java:191)
       at com.github.penfeizhou.animation.FrameAnimationDrawable$1.handleMessage(FrameAnimationDrawable.java:60)
       at android.os.Handler.dispatchMessage(Handler.java:107)
       at android.os.Looper.loop(Looper.java:214)
       at android.app.ActivityThread.main(ActivityThread.java:7366)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:932)