Improve SDP negotiation for mixed encrypted/unencrypted offers.

According to RFC 6904 a header extension MAY be offered both encrypted and unecrypted. In the case when encryption is enabled the encrypted version SHOULD be used and vice versa. However, this is under the assumption that both peers actually offer the same extension header both encrypted and unecrypted. With this PR we tighten the negotiation rules to the encryption option SHOULD be the same both in the sender and receiver in order to not drop the extension. Especially, see test `TestOfferAnswerPreferEncryptedRtpHeaderExtensionsWhenEncryptionEnabled` and `TestOfferAnswerPreferEncryptedRtpHeaderExtensionsWhenEncryptionDisabled`.

Bug: chromium:40623740
Change-Id: I68c65a776fcf7be97aaf60a797594c4361a06800
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/363940
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Emil Vardar (xWF) <vardar@google.com>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43191}