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/SdkConstants.cpp b/tools/aapt2/SdkConstants.cpp
index ebe41da..f7a5ba1 100644
--- a/tools/aapt2/SdkConstants.cpp
+++ b/tools/aapt2/SdkConstants.cpp
@@ -77,9 +77,10 @@
   return iter->second;
 }
 
-Maybe<ApiVersion> GetDevelopmentSdkCodeNameVersion(const StringPiece& code_name) {
+std::optional<ApiVersion> GetDevelopmentSdkCodeNameVersion(const StringPiece& code_name) {
   return (sDevelopmentSdkCodeNames.find(code_name) == sDevelopmentSdkCodeNames.end())
-      ? Maybe<ApiVersion>() : sDevelopmentSdkLevel;
+             ? std::optional<ApiVersion>()
+             : sDevelopmentSdkLevel;
 }
 
 }  // namespace aapt