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/compile/PseudolocaleGenerator.cpp b/tools/aapt2/compile/PseudolocaleGenerator.cpp
index 3f574ee..2461438 100644
--- a/tools/aapt2/compile/PseudolocaleGenerator.cpp
+++ b/tools/aapt2/compile/PseudolocaleGenerator.cpp
@@ -33,7 +33,7 @@
 // The struct that represents both Span objects and UntranslatableSections.
 struct UnifiedSpan {
   // Only present for Span objects. If not present, this was an UntranslatableSection.
-  Maybe<std::string> tag;
+  std::optional<std::string> tag;
 
   // The UTF-16 index into the string where this span starts.
   uint32_t first_char;