Fix broken demo mode

Demo mode was relying on the CollapsedStatusBar's icon manager being the
first element of the IconGroup list, and this is no longer true. Also
demo mode used to be a regular LinearLayout but now needs to be a
StatusIconContainer because of notchiness

Fixes: 73897667
Test: enable demo mode; adb shell am broadcast -a "com.android.systemui.demo" -e command status -e alarm show
Change-Id: Ieb84ac2860082e339305160ac135ee5e78c23d7f