Fix a memory leak in gatekeeper.

In violation to the documentation of GateKeeper::GetAuthTokenKey and
GateKeeper::GetPasswordKey, the implementations in SoftGateKeeper
allocate and return buffers and relinquish ownership causing a memory
leak, because the caller expects the implementation to retain ownership.

Bug: 129768470
Bug: 134557251
Test: gatekeeper-unit-tests
Change-Id: I0af9539d3dcd47dfd1e7d80cdee700ea0c2d6d0f
Merged-In: I0af9539d3dcd47dfd1e7d80cdee700ea0c2d6d0f
(cherry picked from commit 6a9c4e79)