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/java/ManifestClassGenerator.cpp b/tools/aapt2/java/ManifestClassGenerator.cpp
index 09ea03b..a0db41b 100644
--- a/tools/aapt2/java/ManifestClassGenerator.cpp
+++ b/tools/aapt2/java/ManifestClassGenerator.cpp
@@ -19,19 +19,17 @@
 #include <algorithm>
 
 #include "Source.h"
-#include "java/AnnotationProcessor.h"
 #include "java/ClassDefinition.h"
 #include "java/JavaClassGenerator.h"
 #include "text/Unicode.h"
-#include "util/Maybe.h"
 #include "xml/XmlDom.h"
 
 using ::aapt::text::IsJavaIdentifier;
 
 namespace aapt {
 
-static Maybe<std::string> ExtractJavaIdentifier(IDiagnostics* diag, const Source& source,
-                                                const std::string& value) {
+static std::optional<std::string> ExtractJavaIdentifier(IDiagnostics* diag, const Source& source,
+                                                        const std::string& value) {
   std::string result = value;
   size_t pos = value.rfind('.');
   if (pos != std::string::npos) {
@@ -63,7 +61,7 @@
     return false;
   }
 
-  Maybe<std::string> result =
+  std::optional<std::string> result =
       ExtractJavaIdentifier(diag, source.WithLine(el->line_number), attr->value);
   if (!result) {
     return false;