Bug 1079519 - Unable to play video clips during WEBRTC video call

Do not hide the back button under the in use overlay. Handle the case where we try to resume a video while a WebRTC call is in progress

do not hide the pick activity back button under the in-use-error overlay

use a timeupdate event instead of playing event

address review comments
Merge pull request #25908 from davidflanagan/bug1079519-2

Bug 1079519 - Unable to play video clips during WEBRTC video call r=russn,djf