OpenSSLCipher: refactor in preparation for AEAD

BoringSSL uses a different interface for AEAD that is much simplier
called EVP_AEAD. Separate out the EVP_CIPHER usage so that we can have
another subclass with the EVP_AEAD usage.

Bug: 20636336
Change-Id: I661d92bd449f2fcc3c4a6e511155490917ecef0c
2 files changed