webrtcbin: fix picking available payload types

When picking an available payload type, we need to pick one that is available across all media.

The previous code, when multiple media were present, looked at the first one, noticed it had pt 96 as the media pt, then simply looked at the next media, noticed it didn't, and decided 96 was available.

Instead, check if the pt is used by any of the media, if it is, decide it is not available and go to the next pt. I'm fairly sure that was the original intent.