pc: ignore DTLS-decrypted packets in RtpTransport::OnReadPacket

RtpTransport subscribes to the underlying packet transport's read
callback, which fires for both SRTP-bypass packets (kSrtpEncrypted) and
DTLS-decrypted application data (kDtlsDecrypted, e.g. SCTP cleartext).
DcSctpTransport filters on decryption_info(); RtpTransport relied on
InferRtpPacketType to reject the cleartext.

That only works because the default SCTP common header starts with
0x13 0x88 (port 5000), which fails the V=2 check. Any payload whose
first byte has high bits 10 and whose second byte's low 7 bits land in
the RTCP-reserved PT range [64, 96) gets misrouted to SrtpTransport and
fails UnprotectRtcp with a noisy log.

Filter on decryption_info() up front.

Bug: webrtc:517079993
Change-Id: I769a07644eb1286fa6c685cef3e1ef5937545843
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476180
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47830}