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: I8c11b165b34b87e7363a8e96524ea70fe6b28342