SystemUI: fix wrong statusbar background.

If we turned off screen while some panel was visible than
background of PhoneStatusBarView will not be updated because we receiving single
panelExpansionChanged call and after super.panelExpansionChanged mFadingPanel
will be null (onAllPanelsCollapsed cause this).

Allow updating background when mFadingPanel is null.

Change-Id: Ib123f27500790c1b7ec787b7e433372629daa08d