Revert "Switch gatekeeper to std::unique_ptr." Gatekeeper is used in Trusty, which does not support the C++ standard library, so std::unique_ptr is not available. This reverts commit 3b6f0be3850fcf92e6a9097f2042624c7dd80891. Change-Id: Ibe287efc4c68cf3f94abfa7b07dd16bf796ef429