Refactor RTX video codec and payload type assignment
Refactor RTX video codec and payload type assignment

We want to reuse some of this functionality for the new video codec
factories, but not of all it, so this CL refactors out what we want to
reuse to a static function.

BUG=webrtc:7925

Review-Url: https://codereview.webrtc.org/3010743002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19628}

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