Use EVP_AEAD_CTX in crypto/cipher/internal.h.

Change-Id: I3002d95d2b9db4dd05e1c56ef6ae410315b97ab9
Reviewed-on: https://boringssl-review.googlesource.com/4710
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>