Add IceControllerInterface::GetConnections() and fix return type.

This is a replacement for `connections()`, which is const-qualified but
returns a `rtc::ArrayView<const cricket::Connection*>`. Unfortunately,
this is a view of mutable pointers to `const cricket::Connection`
objects: this means that the pointers in `IceControllerInterface`'s
backing store can be reassigned.

Returning `rtc::ArrayView<const cricket::Connection* const>` is more
consistent with how a view returned from a const-qualified method should
behave. Unfortunately, there are downstream overrides of this method, so
instead of just fixing this outright, add a new method with the correct
return type. Once downstream is updated, the old method can then be
removed.

Bug: chromium:1409870, webrtc:15702
Change-Id: I3d8f0c2f0bcf4c393ee63959cb61761ac00a28ab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/329962
Reviewed-by: Jonas Oreland <jonaso@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#41318}