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/Fixture.cpp b/tools/aapt2/test/Fixture.cpp
index 285e5a1..e2f71dc 100644
--- a/tools/aapt2/test/Fixture.cpp
+++ b/tools/aapt2/test/Fixture.cpp
@@ -126,7 +126,7 @@
   link_args.insert(link_args.end(), {"-I", android_sdk});
 
   // Add the files from the compiled resources directory to the link file arguments
-  Maybe<std::vector<std::string>> compiled_files = file::FindFiles(flat_dir, diag);
+  std::optional<std::vector<std::string>> compiled_files = file::FindFiles(flat_dir, diag);
   if (compiled_files) {
     for (std::string& compile_file : compiled_files.value()) {
       compile_file = file::BuildPath({flat_dir, compile_file});