Move atrace policy to private

atrace and its atrace_exec now exist only in private policy.

Test: No change to policy according to sesearch, except for
      disappearance of all allow rules to do with atrace_current
      which is expected now that atrace cannot be referenced from
      public or vendor policy.
Bug: 31364497

Change-Id: Ib726bcf73073083420c7c065cbd39dcddd7cabe3