Closes #785

* Move PluginConfig out of bramble-android. Projects using bramble now need to provide it.
* Add a PluginConfig#shouldPoll() method which can be used to disable polling altogether.
* Move Poller instantiation to the PluginManager.

* Create at least one contact

It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.