Add DeleteUser/DeleteAllUsers to gatekeeper

Not actually implemented yet, but will respond to calls.

Also add "-Werror -Wunused-parameter" to rules.mk, since that's
what's used by presubmit build checks.

Bug: 160731903
Test: "atest VtsHalGatekeeperV1_0TargetTest"
      manual testing with added instrumentation

Change-Id: I3c9c4d3a2d07f4b018cd7b03691b92a777dd9f0d
5 files changed