Support getting external HMAC auth context with libsrtp 2.1.0.
Support getting external HMAC auth context with libsrtp 2.1.0.

This is in preparation of upgrading to libsrtp 2.1.0.

BUG=webrtc:7856

Review-Url: https://codereview.webrtc.org/2958123002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#18805}

ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc