Update paho.mqtt.java dependency to latest version

[loffa]

A lot of fixes has been made in the primary Java-client since it was last updated. Some of these fixes might solve bugs created in the Android lib and should be included as soon as possible.

I suggest that a new release (or at least the next release) should include paho.mqtt.java version 1.2.1 that has now been released.

Created as requested in https://github.com/eclipse/paho.mqtt.java/issues/570#issuecomment-465960641

[chintan-mishra]

Also, add MQTT v5 support in the Android library too.

[hannta]

paho.mqtt.java version 1.2.1 requires Android 7 (API level 24) or later (https://github.com/eclipse/paho.mqtt.java/issues/633)

I think it does not make sense to make such android version requirement for this lib. So before the paho.mqtt.java compatibility issue got solved, there is no way to update this lib IMHO.

[Queatz]

Is there an effort in play to support v5 in the Android service?