SystemUI: Use new USB notifications to detect USB disconnect.

This fixes a bug that prevented the USB mass storage activity from closing
when USB is disconnected.
The bug was actually due to using == for a string compare instead of equals(),
but using the new notifications is a better solution than using battery events
since it will work for devices that do not charge over USB.

BUG: 3018954

Change-Id: Ia447974726a52cd865e59df5af79e828b5134b6c
Signed-off-by: Mike Lockwood <lockwood@android.com>