MQTTv5 - Send disconnect packet when automatically disconnecting

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- [ ] Bug exists Release Version 1.2.0 ( Master Branch)
- [ ] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [X] Bug exists in MQTTv5 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new

This could do with some verification first:

Although pahov5 allows the client application to send a DISCONNECT in some cases before shutting down, it does not do so itself. This is not function which should be left to the application. It is fine for the application to decide whether to continue, but if the client disconnects it should send the DISCONNECT packet first.