[e2e test] Wait for NSSL rather than the notif container

The latest test result of desktop sysui e2e shows that the NSSL can
still be hidden when the notification container shows. Therefore, wait
for the NSSL rather than the notification container when opening the
notification shade.

Bug: 439077019
Flag: TEST_ONLY
Change-Id: I6f6b55ea2933f3df189f7728324b4d5ad60dc650
1 file changed