Fix RTP header extension encryption

Previously, RTP header extensions with encryption had been filtered
if the encryption had been activated (not the other way around) which
was likely an unintended logic inversion.

In addition, it ensures that encrypted RTP header extensions are only
negotiated if RTP header extension encryption is turned on. Formerly,
which extensions had been negotiated depended on the order in which
they were inserted, regardless of whether or not header encryption was
actually enabled, leading to no extensions being sent on the wire.

Further changes:

- If RTP header encryption enabled, prefer encrypted extensions over
  non-encrypted extensions
- Add most extensions to list of extensions supported for encryption
- Discard encrypted extensions in a session description in case encryption
  is not supported for that extension

Note that this depends on https://github.com/cisco/libsrtp/pull/491 to get
into libwebrtc (cherry-pick or bump libsrtp version). Otherwise, two-byte
header extensions will prevent any RTP packets being sent/received.

Bug: webrtc:11713
Change-Id: Ia0779453d342fa11e06996d9bc2d3c826f3466d3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177980
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Taylor <deadbeef@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33723}

modules/rtp_rtcp/source/rtp_packet.cc

@@ -27,6 +27,7 @@ constexpr size_t kFixedHeaderSize = 12;
 constexpr uint8_t kRtpVersion = 2;
 constexpr uint16_t kOneByteExtensionProfileId = 0xBEDE;
 constexpr uint16_t kTwoByteExtensionProfileId = 0x1000;
+constexpr uint16_t kTwobyteExtensionProfileIdAppBitsFilter = 0xfff0;
 constexpr size_t kOneByteExtensionHeaderLength = 1;
 constexpr size_t kTwoByteExtensionHeaderLength = 2;
 constexpr size_t kDefaultPacketSize = 1500;
@@ -501,7 +502,8 @@ bool RtpPacket::ParseBuffer(const uint8_t* buffer, size_t size) {
       return false;
     }
     if (profile != kOneByteExtensionProfileId &&
-        profile != kTwoByteExtensionProfileId) {
+        (profile & kTwobyteExtensionProfileIdAppBitsFilter) !=
+            kTwoByteExtensionProfileId) {
       RTC_LOG(LS_WARNING) << "Unsupported rtp extension " << profile;
     } else {
       size_t extension_header_length = profile == kOneByteExtensionProfileId