Replace Maybe with std::optional

With c++17, std::optional provides the functionality that Maybe
provided.

Bug: 183215655
Test: aapt2_tests
Change-Id: I62bb9c2fa4985dc5217a6ed153df92b85ad9a034
diff --git a/tools/aapt2/cmd/Convert.h b/tools/aapt2/cmd/Convert.h
index 7e2029d..2cdb0c8 100644
--- a/tools/aapt2/cmd/Convert.h
+++ b/tools/aapt2/cmd/Convert.h
@@ -17,6 +17,8 @@
 #ifndef AAPT2_CONVERT_H
 #define AAPT2_CONVERT_H
 
+#include <optional>
+
 #include "Command.h"
 #include "LoadedApk.h"
 #include "format/binary/TableFlattener.h"
@@ -52,7 +54,7 @@
   TableFlattenerOptions table_flattener_options_;
   XmlFlattenerOptions xml_flattener_options_;
   std::string output_path_;
-  Maybe<std::string> output_format_;
+  std::optional<std::string> output_format_;
   bool verbose_ = false;
 };