videoprovider: remove frame subscription on QVideoSink::destroyed

We can observe that frame update signals can occur after a VideoOutput component's QVideoSink object has started destruction. The deregistration of the object pointer needs to be synchronized to the object's destruction.

Additionally:
+ scope the frame obj mutex locker around checks for no subs
+ rename (un)registerSink to (un)subscribe
+ subscribe will now remove the previous subscription
+ subscribe with an empty id will unsubscribe

Gitlab: #997
Change-Id: I21460564e49189b9276f153e8c1fd3a5cc52c6b2