Use optional for nullable types

AIDL generates optional<T> for nullable T types for C++, which is more
efficient and idomatic and easy to use.

Bug: 144773267
Test: build/flash/boot
Merged-In: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
Change-Id: I90cf2ce1193c479179687d71a5c1416f6cdf0b16
(cherry picked from commit ff7f4279bd7fa33577f5f4d46aa9de322fb625d4)

Exempt-From-Owner-Approval: CP from master
diff --git a/cmds/idmap2/idmap2d/Idmap2Service.cpp b/cmds/idmap2/idmap2d/Idmap2Service.cpp
index 8ee79f6..d723776 100644
--- a/cmds/idmap2/idmap2d/Idmap2Service.cpp
+++ b/cmds/idmap2/idmap2d/Idmap2Service.cpp
@@ -113,10 +113,10 @@
 Status Idmap2Service::createIdmap(const std::string& target_apk_path,
                                   const std::string& overlay_apk_path, int32_t fulfilled_policies,
                                   bool enforce_overlayable, int32_t user_id ATTRIBUTE_UNUSED,
-                                  std::unique_ptr<std::string>* _aidl_return) {
+                                  std::optional<std::string>* _aidl_return) {
   assert(_aidl_return);
   SYSTRACE << "Idmap2Service::createIdmap " << target_apk_path << " " << overlay_apk_path;
-  _aidl_return->reset(nullptr);
+  _aidl_return->reset();
 
   const PolicyBitmask policy_bitmask = ConvertAidlArgToPolicyBitmask(fulfilled_policies);
 
@@ -155,7 +155,7 @@
     return error("failed to write to idmap path " + idmap_path);
   }
 
-  *_aidl_return = std::make_unique<std::string>(idmap_path);
+  *_aidl_return = idmap_path;
   return ok();
 }