Revert "Privatize sigslots in NetworkManager"

This reverts commit 12168786cc61f58792acaa02ac917aa1ea996801.

Reason for revert: Missed a reference in Chromium

Original change's description:
> Privatize sigslots in NetworkManager
>
> To be submitted after Chromium adaptations (now landed):
> https://chromium-review.googlesource.com/c/chromium/src/+/7172038
>
> Bug: webrtc:42222066
> Change-Id: Ic3f0d2c717bc38dc13db4e1b8cd6e788bfc1df7c
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/426980
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46262}

Bug: webrtc:42222066
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I7c04eaaafa6a403ba1253cec72e18c904c64e564
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427061
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46272}