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/cmd/Command.cpp b/tools/aapt2/cmd/Command.cpp
index 919b4c9..b1452fa 100644
--- a/tools/aapt2/cmd/Command.cpp
+++ b/tools/aapt2/cmd/Command.cpp
@@ -72,7 +72,7 @@
 }
 
 void Command::AddOptionalFlag(const StringPiece& name, const StringPiece& description,
-                              Maybe<std::string>* value, uint32_t flags) {
+                              std::optional<std::string>* value, uint32_t flags) {
   auto func = [value, flags](const StringPiece& arg) -> bool {
     *value = (flags & Command::kPath) ? GetSafePath(arg) : arg.to_string();
     return true;