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/process/SymbolTable.h b/tools/aapt2/process/SymbolTable.h
index 06eaf63..65ae7be 100644
--- a/tools/aapt2/process/SymbolTable.h
+++ b/tools/aapt2/process/SymbolTable.h
@@ -56,7 +56,7 @@
   struct Symbol {
     Symbol() = default;
 
-    explicit Symbol(const Maybe<ResourceId>& i, const std::shared_ptr<Attribute>& attr = {},
+    explicit Symbol(const std::optional<ResourceId>& i, const std::shared_ptr<Attribute>& attr = {},
                     bool pub = false)
         : id(i), attribute(attr), is_public(pub) {
     }
@@ -66,7 +66,7 @@
     Symbol& operator=(const Symbol&) = default;
     Symbol& operator=(Symbol&&) = default;
 
-    Maybe<ResourceId> id;
+    std::optional<ResourceId> id;
     std::shared_ptr<Attribute> attribute;
     bool is_public = false;
     bool is_dynamic = false;