Ignore internal transport packets in DatagramConnection

Prevent DatagramConnection from forwarding internal and post-handshake
packets (such as BoringSSL session tickets, DTLS handshakes, or STUN
connectivity checks) to the OnSendOutcome observer. These internal
packets are assigned a default packet ID of -1, signaling that they
are not user-initiated data.

This could cause the following error:

[ RUN ]
DatagramConnectionTest.RtpPacketsAreSent
../../pc/datagram_connection_unittest.cc:219: Failure Mock function
called more times than expected - returning directly.
    Function call: OnSendOutcome
         Expected: to be called once
           Actual: called twice - over-saturated and active
[ FAILED ] DatagramConnectionTest.RtpPacketsAreSent (108 ms)

Additionally, update the `Observer::SendOutcome` initialization in
OnSentPacket and DispatchSendOutcome to use structured initialization,
and Add a fallback to Timestamp::MinusInfinity() and avoid setting the
timestamp to -1ms.

Bug: none
Change-Id: I6576110ec5f9aa620b8870d3f6db963d1f5e2bbd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/477140
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#47862}