Fix NPE when QSPanel not yet attached,

The NotificationPanelViewController tries to use the
QSPanelController before the QSPanel has been attached (sometimes).

Handle the case where it tries to set the expansion pre-attachment
and add a corresponding test.

Fixes: 172520720
Test: atest SystemUITests
Change-Id: Ib957201760de21561df59ec6e4f459ae7bcbf828