Revert "system/gatekeeper: use proper nativehelper headers"

This reverts commit 05b35791c3079543c680efba0789f03cfddafacb.

...
In file included from system/gatekeeper/gatekeeper.cpp:16:
In file included from system/gatekeeper/include/gatekeeper/gatekeeper.h:22:
libnativehelper/include/nativehelper/UniquePtr.h:20:10: fatal error: 'cstdlib' file not found
#include <cstdlib> // For NULL.

Exempt-From-Owner-Approval: build breakage

Change-Id: Ib6e181046504b2559093ce6d92d9d701a527ba01
diff --git a/gatekeeper.cpp b/gatekeeper.cpp
index 76fc8d6..44993cf 100644
--- a/gatekeeper.cpp
+++ b/gatekeeper.cpp
@@ -13,8 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#include <UniquePtr.h>
 #include <gatekeeper/gatekeeper.h>
-#include <nativehelper/UniquePtr.h>
 
 #include <endian.h>