Fix the way to remove unsubscribed channels to avoid ConcurrentModificationException

[kikuchy]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix

⤵️ What is the current behavior?

Pusher.init() on Android will cause ConcurrentModificationException when the app will be Hot Restarted.

It comes from the removing Iterator elements in for-in statement.
e.g.) https://stackoverflow.com/questions/18448671/how-to-avoid-concurrentmodificationexception-while-removing-elements-from-arr/18448699

To avoid that, I move the deletion of channels to after the for-in statement of init().

🆕 What is the new behavior (if this is a feature change)?

No new behavior.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

📝 Links to relevant issues/docs

Stack trace about the bug. You can see it when you hot restart the app on Android.

I/flutter (22977): PlatformException(error, null, null, java.util.ConcurrentModificationException
I/flutter (22977):  at java.util.HashMap$HashIterator.nextNode(HashMap.java:1441)
I/flutter (22977):  at java.util.HashMap$EntryIterator.next(HashMap.java:1475)
I/flutter (22977):  at java.util.HashMap$EntryIterator.next(HashMap.java:1473)
I/flutter (22977):  at com.ninjasolutions.pusher.PusherPlugin.init(PusherPlugin.java:117)
I/flutter (22977):  at com.ninjasolutions.pusher.PusherPlugin.onMethodCall(PusherPlugin.java:86)
I/flutter (22977):  at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:233)
I/flutter (22977):  at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
I/flutter (22977):  at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:692)
I/flutter (22977):  at android.os.MessageQueue.nativePollOnce(Native Method)
I/flutter (22977):  at android.os.MessageQueue.next(MessageQueue.java:326)
I/flutter (22977):  at android.os.Looper.loop(Looper.java:160)
I/flutter (22977):  at android.app.ActivityThread.main(ActivityThread.java:6669)
I/flutter (22977):  at java.lang.reflect.Method.invoke(Native Method)
I/flutter (22977):  at com.android.internal.os.RuntimeInit$MethodAndArgsCal
I/flutter (22977): 
I/flutter (22977): #0      StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:582:7)
I/flutter (22977): #1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:159:18)
I/flutter (22977): <asynchronous suspension>
I/flutter (22977): #2      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:332:12)
I/flutter (22977): #3      Pusher.init (package:flutter_pusher/pusher.dart:49:20)

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[genert]

LGTM