AWSIotMqttManager messagequeue

[rvp-thunderbuild]

Occasionally the AWSIotMqttManager throws the following exception on the main thread:
java.lang.NullPointerException: Attempt to read from field 'java.lang.Object java.util.LinkedList$Node.item' on a null object reference at java.util.LinkedList.unlink(LinkedList.java:211) at java.util.LinkedList.remove(LinkedList.java:526) at com.amazonaws.mobileconnectors.iot.AWSIotMqttManager.publishMessagesFromQueue(AWSIotMqttManager.java:1224)

Which will cause the entire app to crash.
This was reported earlier in #421 and never fixed.

I haven't been able to consistently reproduce this issue because it seems to be timing related where multiple threads are removing items from the message queue. In order to solve this problem I propose the following two fixes:

1. Replace the LinkedList with a ConcurrentLinkedQueue in order to allow better access for multiple threads.
2. Use a looper of a thread different from the main thread, since there is absolutely no need to push queued message from the main thread.

[mutablealligator]

Approving the change to move from LinkedList to ConcurrentLinkedQueue. The HandlerThread->Looper change will be done separately.

[rvp-thunderbuild]

@kvasukib When will the Looper issue be addressed? And where can I follow this issue?

[mutablealligator]

@rvp-thunderbuild For the looper change, we need to ensure that the handler thread quits gracefully. Otherwise, there will be a lot of threads lying around without being garbage collected. You are welcome to submit a PR with the additional change of quitting the handler thread appropriately or open a new issue. Thanks.