Use absl::AnyInvocable instead of std::function in RtpTransmissionManager/DtlsSrtpTransport

This change updates `RtpTransmissionManager` and `DtlsSrtpTransport`
to use `absl::AnyInvocable<void()>` instead of `std::function<void()>`.
This includes updating both the header and the implementation files.

This aligns with webrtc guidance:
https://webrtc.googlesource.com/src/+/refs/heads/main/g3doc/style-guide.md

This change was made by Gemini with the following prompt:
Use commit d471f7cf9a3e7900404ba5ef1 as a template for how to port each use of std::function in the `@pc/rtp_transmission_manager.h` and `@pc/dtls_srtp_transport.h` files to absl::AnyInvocable. Use absl::AnyInvocable<void() &&> for callbacks used once and absl::AnyInvocable<void()> otherwise. Try to compile your changes iteratively. When done, write a commit message in the same style as the commit above and acknowledge this was done by gemini and include this prompt. Make sure that the git commit message looks as you intended it to look.

No-IWYU: Bot still broken
Bug: None
Change-Id: I3c0a81728bb7d4df9a59cef6f591aca72464b332
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/425101
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46198}