main/bridge: Add some verbose logging for video source changes

It's actually quite useful to see the source of a video stream change.
This doesn't happen terribly often, even with talk detection - but when
it does, it's nice to know which channel is now providing your video
stream.

As a verbose 5 level message, it shouldn't be terribly spammy or costly
to have, and is 'lower level' then most other verbose messages that the
bridge system emits.

ASTERISK-26555

Change-Id: Ia1c20ecafa9670171fd38bddcf3beccae47fb15c