SetTimer is a continuous timer, but we want to use it as a
one-shot, so make sure we actually kill it when not needed.

Change-Id: If6f5374346dc530eb664545ade2c2955885941b5
Reviewed-on: https://gerrit.libreoffice.org/59589
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>