Use aidl::nullable for nullable type in C++

To reduce the discrepance between old code which still uses
std::unique_ptr and new code using std::optional.

This might help to avoid merge-conflicts between branches.

Bug: 144773267
Test: m
Merged-In: Ie3196ee5cce17d77950eea9479d2cc1406e9e674
Merged-In: I33822bc76ef87637d5408849f64a0607e121792e
Change-Id: I33822bc76ef87637d5408849f64a0607e121792e
(cherry picked from commit ad62e8cbf5cf6083568f6f230ef7d73ad8776971)

Exempt-From-Owner-Approval: cherry-pick from master with owner's approval
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.h b/cmds/idmap2/idmap2d/Idmap2Service.h
index 94d2af4..b6f5136 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.h
+++ b/cmds/idmap2/idmap2d/Idmap2Service.h
@@ -19,9 +19,7 @@
 
 #include <android-base/unique_fd.h>
 #include <binder/BinderService.h>
-
-#include <memory>
-#include <string>
+#include <binder/Nullable.h>
 
 #include "android/os/BnIdmap2.h"
 
@@ -46,7 +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,
-                             std::unique_ptr<std::string>* _aidl_return) override;
+                             aidl::nullable<std::string>* _aidl_return) override;
 };
 
 }  // namespace android::os