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.

bramble-j2se/src/main/java/org/briarproject/bramble/plugin/DesktopPluginModule.java

@@ -59,6 +59,11 @@ public class DesktopPluginModule extends PluginModule {
 			public Collection<SimplexPluginFactory> getSimplexFactories() {
 				return simplex;
 			}
+
+			@Override
+			public boolean shouldPoll() {
+				return true;
+			}
 		};
 		return pluginConfig;
 	}