One one hand, this fixes missing invalidations after typing characters
in editeng on Android. Previously it was assumed that there is only one
sd::ViewShell for one Impress document, but that's obviously not true.

On the other hand, this will be handy when later svx code wants to
invoke the LOK callback as well, when it was stored in sd, that wasn't
possible.

Change-Id: Id467be01ad008aecaaabdd85b2a6b29a14f1eb86