Reduce network cost for cellular network slices.

This change makes network slices on cellular networks more preferable
than the regular network by decreasing the network cost slightly when a
slice is in use.

Network slicing is typically offered as a premium feature on 5G networks
at an additional price.

The network cost is only reduced when the underlying adapter supports
slicing, currently 5G or undifferentiated cellular. It is gated behind
the WebRTC-UnifiedCommunications field trial.

BUG=webrtc:466507512

Change-Id: I230264c95d2d9f0386a096e7ca51cf44b5684922
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442681
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Sameer Vijaykar <samvi@google.com>
Cr-Commit-Position: refs/heads/main@{#46710}