@Override
                public CharSequence flatten(long timeMillis, int logLevel, String tag, String message) {
                    return flattener.flatten(logLevel, tag, message);
                }
Set the flattener when print a log.

@param flattener the flattener when print a log
@return the builder
@deprecated {@link Flattener} is deprecated, use {@link #flattener(Flattener2)} instead,
since 1.6.0
android/android_framework/base/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java