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/util/Files.h b/tools/aapt2/util/Files.h
index 481a4cd..877cd56 100644
--- a/tools/aapt2/util/Files.h
+++ b/tools/aapt2/util/Files.h
@@ -18,6 +18,7 @@
 #define AAPT_FILES_H
 
 #include <memory>
+#include <optional>
 #include <string>
 #include <unordered_set>
 #include <vector>
@@ -27,7 +28,6 @@
 #include "utils/FileMap.h"
 
 #include "Diagnostics.h"
-#include "Maybe.h"
 #include "Source.h"
 
 namespace aapt {
@@ -43,7 +43,7 @@
 
 enum class FileType {
   kUnknown = 0,
-  kNonexistant,
+  kNonExistant,
   kRegular,
   kDirectory,
   kCharDev,
@@ -81,7 +81,7 @@
 std::string PackageToPath(const android::StringPiece& package);
 
 // Creates a FileMap for the file at path.
-Maybe<android::FileMap> MmapPath(const std::string& path, std::string* out_error);
+std::optional<android::FileMap> MmapPath(const std::string& path, std::string* out_error);
 
 // Reads the file at path and appends each line to the outArgList vector.
 bool AppendArgsFromFile(const android::StringPiece& path, std::vector<std::string>* out_arglist,
@@ -124,8 +124,9 @@
 
 // Returns a list of files relative to the directory identified by `path`.
 // An optional FileFilter filters out any files that don't pass.
-Maybe<std::vector<std::string>> FindFiles(const android::StringPiece& path, IDiagnostics* diag,
-                                          const FileFilter* filter = nullptr);
+std::optional<std::vector<std::string>> FindFiles(const android::StringPiece& path,
+                                                  IDiagnostics* diag,
+                                                  const FileFilter* filter = nullptr);
 
 }  // namespace file
 }  // namespace aapt