Reduce observers in SystemUI: StatusBarIconView

Currently, for each status bar icon view there is a observer that
listens for changes of STATUS_BAR_NOTIF_COUNT. With one
notification usually having one or more status bar icons
you may end up easily with 10+ more observers for ~5 notifications.

With this commit the internal observer class becomes a singleton.
This means there is now only one observer for all status bar icons
and additional binder calls on new notification are avoided.

See "dumpsys content"

Change-Id: I7ca6481fe43e87c34b19d7b71e531cce93cf80ef