Store SRTP_PROTECTION_PROFILES as const.

They're small, but they should be read-only. This slightly changes public API
and affects downstream WebRTC code.

Hold on landing this until https://webrtc-codereview.appspot.com/34649004/
rolls into Chromium.

Change-Id: I93cbae20f69d55411d6b1cb62ed7d9a81c83b701
Reviewed-on: https://boringssl-review.googlesource.com/2720
Reviewed-by: Adam Langley <agl@google.com>