Disconnected Buffer is not cleared when cleansession is true

Hi, I am using Paho library for my android app and I am facing an issue related to offline buffering of messages.

When cleansession is true, `MqttClientPersistence` is cleared when client reconnects but the disconnected buffer is not cleared. This creates inconsistent behaviour.
- When client reconnects in the same session, messages are sent from the disconnected buffer
- When client reconnects on process restart, no messages are sent as the persistence is cleared due to cleansession.

- [x] Bug exists Release Version 1.2.5 ( Master Branch)
- [x] Bug exists in MQTTv3 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch)
- [x] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-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

Start by tracing the MQTTv3 and MQTTv5 reconnect paths that handle the disconnected buffer and MqttClientPersistence. Reproduce both cases described: reconnect in the same session and reconnect after a process restart with cleansession true; done means the disconnected buffer is cleared consistently with persistence.

Written by the indexing model from the issue text.