Add debug text to NotificationPanelViewController

This fixes a bug where setting MULTIUSER_DEBUG to true would result in a
NullPointerException. It also deletes an invisible TextView from
quick_status_bar_expanded_header.xml.

Test: Set MULTIUSER_DEBUG to true in StatusBar
Bug: 169783558
Change-Id: I357346449ade9a70ca935fcffaee03f12c7400d6