SystemUI: fix memory leaks

ContentObservers are being registered without being unregistered,
causing old objects not to be garbage collected. This causes memory
leak if the classes are destroyed and recreated throughout the
device's uptime.

StatusBarIconView is created every time a notification is triggered.

JIRA: CYAN-650
Change-Id: Iabd3221fc2b0c848396af1790fd09424be602e7e