Check that createLogHandler() isn't called more than once.

This would result in multiple log files encrypted with the same key.

briar-api/src/main/java/org/briarproject/briar/api/logging/PersistentLogManager.java

@@ -24,6 +24,8 @@ public interface PersistentLogManager {
 	/**
 	 * Creates and returns a persistent log handler that stores its logs in
 	 * the given directory.
+	 * <p>
+	 * This method should only be called once.
 	 */
 	Handler createLogHandler(File dir) throws IOException;