Do not allow printing until preview is updated

Otherwise we might end up in the situation where we print something that
was not previewed.
This is of course bad for the user as he prints something he has not seen.
It is also bad for the print spooler as in the case the layout did not
describe the amount of pages, we do not know the number of pages available
until the update is complete.

We now allow changing of printer at any time unless printing is in final
stage. If we don't allow this the changing of printer would be blocked
until the intial document was written which might take some time.

Fixes: 36599750
Test: cts-tradefed run cts-dev -m Print
Change-Id: I93e910c02f2a770008b845028f0adf17b3d410e2