Use non-proxied source object in VideoTrack.

Use the internal representation of the video source object from the
track. Before there were implicit thread hops due to use of the proxy.

Also, override AudioTrack's enabled methods to enforce thread
expectations.

Bug: webrtc:13540
Change-Id: I4bc7aca96d6fc24f31ade45e47f52599f1cc2f97
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250180
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35911}