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This change simply switches from the deprecated
EVP_{En|De}crypt{Init|Final} to the newer, _ex versions of the same.

There is no difference in behaviour, save for calling
EVP_CIPHER_CTX_init, as the deprecated versions are just wrappers around
the _ex functions. See
https://git.openssl.org/gitweb/?p=openssl.git;a=blob;f=crypto/evp/evp_enc.c;h=f705967a40ab92cdf3c2ba8dd6bc19680d6157d6;hb=HEAD#l274



This change is required for the transition to BoringSSL, which removes
the deprecated functions.

Bug: 17409664
Change-Id: I35c6cc2d86d0c876a9edaff1e5571170fe393d87
Signed-off-by: Adam Langley <agl@google.com>