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/Main.cpp b/tools/aapt2/Main.cpp
index 8a43bb4..b249c6c 100644
--- a/tools/aapt2/Main.cpp
+++ b/tools/aapt2/Main.cpp
@@ -147,7 +147,7 @@
  private:
   io::FileOutputStream* out_;
   IDiagnostics* diagnostics_;
-  Maybe<std::string> trace_folder_;
+  std::optional<std::string> trace_folder_;
 };
 
 }  // namespace aapt