OpenSSLCipher: add AEAD cipher

This allows us to provide an implementation of AES-GCM using the new
EVP_AEAD interface in BoringSSL. It simply buffers up the input until
doFinal(...) is called which makes it much safer than any streaming
interfaces, because the caller can't use the plaintext until it's
authenticated by the GHASH (or whatever other AEAD you happen to use).

Bug: 20636336
Change-Id: I6e4b063a8137a16102b1f6ac15687a38ddfe1691
5 files changed