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/Context.h b/tools/aapt2/test/Context.h
index 5d8ded3..e1b8dd5 100644
--- a/tools/aapt2/test/Context.h
+++ b/tools/aapt2/test/Context.h
@@ -95,8 +95,8 @@
   friend class ContextBuilder;
 
   PackageType package_type_ = PackageType::kApp;
-  Maybe<std::string> compilation_package_;
-  Maybe<uint8_t> package_id_;
+  std::optional<std::string> compilation_package_;
+  std::optional<uint8_t> package_id_;
   StdErrDiagnostics diagnostics_;
   NameMangler name_mangler_;
   SymbolTable symbols_;