public Builder flattener(Flattener2 flattener) {
            this.flattener = flattener;
            return this;
        }
Set the flattener when print a log.

@param flattener the flattener when print a log
@return the builder
@since 1.6.0
android/android_framework/base/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java