Don't NPE in PanelView

AUPT seems to have found that this can happen sometimes;
PanelView#notifyBarPanelExpansionChanged() gets called before mBar is
set. So just don't call methods on it in that case

Fixes: 124002167
Test: atest SystemUITests
Change-Id: I137b416c3cfabd8f041c4867b4a4f25a483f00d4