Add method for getting reason why plugin is disabled.

bramble-java/src/main/java/org/briarproject/bramble/plugin/modem/ModemPlugin.java

@@ -124,6 +124,11 @@ class ModemPlugin implements DuplexPlugin, Modem.Callback {
 		return state.getState();
 	}
 
+	@Override
+	public int getReasonDisabled() {
+		return getState() == DISABLED ? REASON_STARTING_STOPPING : -1;
+	}
+
 	@Override
 	public boolean shouldPoll() {
 		return false;