Fixed a bug where autobundling could lead to bugs

Notifications could appear twice if a notification with an app
group became bundled and one could not be swiped away anymore.
This is because we were unconditionally copying over the override
key even if the notification just became a group.

This also fixes a nullpointer that could happen with a racecondition
when a notification was cancelled and it tried to be used for autobundling.

This also fixes an issue that the autobundling wasn't working correctly
when a notification didn't have a group set but was set to be a
group summary.

Change-Id: Icd971f16ae0804ce162fd3171fcdd99bd04f3885
Fixes: 28706404
Fixes: 28628237