Instead, add a separate init_with_direction hook. Normal AEADs ignore the
direction, while legacy AEADs must be initialized with it. This avoids
maintaining extra state to support the delayed initialization.
Change-Id: I25271f0e56ee2783a2fd4d4026434154d58dc0a8
Reviewed-on: https://boringssl-review.googlesource.com/3731
Reviewed-by: Adam Langley <agl@google.com>