QS: Don't assume observers are registered when dealing with changes

Not all uri activity has an associated content observer. Poking
the content://settings/system provider manually, for example,
would result in the following NPE:

AndroidRuntime: FATAL EXCEPTION: main
AndroidRuntime: java.lang.NullPointerException
AndroidRuntime:        at com.android.systemui.statusbar.phone.QuickSettingsController$QuickSettingsObserver.onChange(QuickSettingsController.java:354)
AndroidRuntime:        at android.database.ContentObserver$NotificationRunnable.run(ContentObserver.java:180)
AndroidRuntime:        at android.os.Handler.handleCallback(Handler.java:725)

Change-Id: I7034d261804aa972fe9020dd9b602348d249cb33