Make plugins and polling configurable
- 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.
These changes will allow the repeater to use the bramble-android module without polling and provides the option to choose which transport plugins should be used.