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/gatekeeperd.cpp b/gatekeeperd/gatekeeperd.cpp
index 184c6d2..61c8804 100644
--- a/gatekeeperd/gatekeeperd.cpp
+++ b/gatekeeperd/gatekeeperd.cpp
@@ -23,6 +23,7 @@
 #include <inttypes.h>
 #include <stdint.h>
 #include <unistd.h>
+#include <memory>
 
 #include <binder/IPCThreadState.h>
 #include <binder/IServiceManager.h>
@@ -383,7 +384,7 @@
 
 private:
     sp<IGatekeeper> hw_device;
-    UniquePtr<SoftGateKeeperDevice> soft_device;
+    std::unique_ptr<SoftGateKeeperDevice> soft_device;
 
     bool clear_state_if_needed_done;
 };