Fix a print spooler crash when printing.

The spooler communicates with he remote PDF renderer asynchronously.
When print is confirmed we close the renderer, destroy it, and unbind
from its service. If we unbind from the service after the print activiy
is finished we get a crash. The bug was that we did not wait until we
disconnect from the remote renderer before finishing the print activity.

bug:17583115

Change-Id: I55b0135f9c5658b3a4fda2901b8b3bdef044e211