Reland "Notify first packet received after receptiveness change"

This is a reland of commit a14f6dc908f5e23c2b58062f5ae3bfbd7aec8502


PS1: original change
PS2..3: add default to Java observer
PS4..5: use absl::AnyInvocable instead of std::function

Original change's description:
> Notify first packet received after receptiveness change
>
> similar to NotifyFirstPacket but fires after changes to
> the receptiveness.
>
> No-IWYU: bot broken
> Bug: chromium:40821064
> Change-Id: I9fc9a229a13891dee2f286faadb8a53e54cd370d
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/411380
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Cr-Commit-Position: refs/heads/main@{#46158}


No-IWYU: bot broken
Bug: chromium:40821064
Change-Id: Ibf7cca9074a2657bb31915959afbfa0b3cc3d8c4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/424880
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#46177}