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/Diff.cpp b/tools/aapt2/cmd/Diff.cpp
index 3950f33..d9e8c92 100644
--- a/tools/aapt2/cmd/Diff.cpp
+++ b/tools/aapt2/cmd/Diff.cpp
@@ -87,8 +87,8 @@
 }
 
 template <typename Id>
-static bool IsIdDiff(const Visibility::Level& level_a, const Maybe<Id>& id_a,
-                     const Visibility::Level& level_b, const Maybe<Id>& id_b) {
+static bool IsIdDiff(const Visibility::Level& level_a, const std::optional<Id>& id_a,
+                     const Visibility::Level& level_b, const std::optional<Id>& id_b) {
   if (level_a == Visibility::Level::kPublic || level_b == Visibility::Level::kPublic) {
     return id_a != id_b;
   }