gatekeeperd: use std::unique_ptr

std::unique_ptr is available in this scope. Substitue the UniquePtr to
std::unique_ptr.

Bug: 63686260
Test: build and boot
Change-Id: Ib8ea3fb5c49c0e92d962f65f1139b073168f8698
diff --git a/gatekeeperd/SoftGateKeeper.h b/gatekeeperd/SoftGateKeeper.h
index 92d1752..2f4f4d7 100644
--- a/gatekeeperd/SoftGateKeeper.h
+++ b/gatekeeperd/SoftGateKeeper.h
@@ -27,10 +27,10 @@
 
 #include <android-base/memory.h>
 #include <gatekeeper/gatekeeper.h>
-#include <nativehelper/UniquePtr.h>
 
 #include <iostream>
 #include <unordered_map>
+#include <memory>
 
 namespace gatekeeper {
 
@@ -173,7 +173,7 @@
     typedef std::unordered_map<uint32_t, failure_record_t> FailureRecordMap;
     typedef std::unordered_map<uint64_t, fast_hash_t> FastHashMap;
 
-    UniquePtr<uint8_t[]> key_;
+    std::unique_ptr<uint8_t[]> key_;
     FailureRecordMap failure_map_;
     FastHashMap fast_hash_map_;
 };