Refactor: Move ssrc, media_channel, SetFrameDecryptor, and SetFrameTransformer to RtpReceiverBase

- Add media_channel() pure virtual getter to RtpReceiverInternal
- Move ssrc() implementation and signaled_ssrc_ member to RtpReceiverBase
- Move SetFrameTransformer/frame_transformer_ to RtpReceiverBase
- Move SetFrameDecryptor/GetFrameDecryptor/frame_decryptor_ to RtpReceiverBase
- Add media_channel() override to AudioRtpReceiver and VideoRtpReceiver

Both concrete receivers had identical implementations of these methods.
Consolidating them in the base class reduces duplication and provides a
foundation for wiring SFrame decryption at the base class level.

Bug: webrtc:479862368
Change-Id: I8c2769006820e683d06491522b31ba321e975d16
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/466421
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47550}