[BUG] Frozen camera if it's turn off

[NghiaTranUIT]

Description

If the current user turns off the video -> The other member will see a frozen image. It's a bug and we should fix it

Acceptance Criteria

• If a video is turned off, replace it with a gray background

Ref: https://github.com/NghiaTranUIT/COSC2769-rmit/pull/68#pullrequestreview-730481096