Cache clientConnection in SurfaceInterface

Order of a client teardown is:
 - ClientConnection is removed from the static map
 - All our client owned resources are torn down
 - ClientConnection is deleted (via a previous deleteLater)

The recent refactor led to a behavioural change where ::client could
return a null pointer.

We want the client getter to be valid throughout the lifespan of
SurfaceInterface, by doing the lookup once we achieve that.

BUG: 424255