[codec_impl] Make DecryptorAdapter::Decrypt and arguments public

For unit testing, the method DecryptorAdapter::Decrypt as well as the
arguments needed to call it are moved from protected to public.

Change-Id: Ia58b6f82083441c9ed2e248e344ec5f8a2207daf
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/556284
Reviewed-by: RJ Ascani <rjascani@google.com>
Commit-Queue: Jonathan Bailey <jonathanbailey@google.com>