Switch aidl::nullable to std::optional

AIDL's @nullable is represented as std::optional in C++ backend.
The previous aidl::nullable was a temporary solution to avoid
merge-conflicts between rvc-dev and master branches.

Bug: 151275535
Test: m
Change-Id: I8fe5bfc1763087e7ff364b4408f862436492d1db
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.h b/cmds/idmap2/idmap2d/Idmap2Service.h
index 55fb5ad..abee999 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.h
+++ b/cmds/idmap2/idmap2d/Idmap2Service.h
@@ -19,7 +19,6 @@
 
 #include <android-base/unique_fd.h>
 #include <binder/BinderService.h>
-#include <binder/Nullable.h>
 
 #include "android/os/BnIdmap2.h"
 
@@ -45,7 +44,7 @@
   binder::Status createIdmap(const std::string& target_apk_path,
                              const std::string& overlay_apk_path, int32_t fulfilled_policies,
                              bool enforce_overlayable, int32_t user_id,
-                             aidl::nullable<std::string>* _aidl_return) override;
+                             std::optional<std::string>* _aidl_return) override;
 
  private:
   // Cache the crc of the android framework package since the crc cannot change without a reboot.