The DAGScheduler was sending a "stage submitted" event before it properly
updated the event's information. This meant that a listener (e.g. the
even logging listener) could record wrong information about the event.

This change sets the stage's submission time before the event is submitted,
when there are tasks to be executed in the stage.

Tested with existing unit tests.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #17925 from vanzin/SPARK-20205.