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/test/Builders.cpp b/tools/aapt2/test/Builders.cpp
index 4816596..23331de 100644
--- a/tools/aapt2/test/Builders.cpp
+++ b/tools/aapt2/test/Builders.cpp
@@ -159,7 +159,8 @@
   return std::move(table_);
 }
 
-std::unique_ptr<Reference> BuildReference(const StringPiece& ref, const Maybe<ResourceId>& id) {
+std::unique_ptr<Reference> BuildReference(const StringPiece& ref,
+                                          const std::optional<ResourceId>& id) {
   std::unique_ptr<Reference> reference = util::make_unique<Reference>(ParseNameOrDie(ref));
   reference->id = id;
   return reference;
@@ -218,7 +219,8 @@
   return std::move(style_);
 }
 
-StyleableBuilder& StyleableBuilder::AddItem(const StringPiece& str, const Maybe<ResourceId>& id) {
+StyleableBuilder& StyleableBuilder::AddItem(const StringPiece& str,
+                                            const std::optional<ResourceId>& id) {
   styleable_->entries.push_back(Reference(ParseNameOrDie(str)));
   styleable_->entries.back().id = id;
   return *this;