HeadsUp: Only remove heads up if attached to window

Attempting to remove mHeadsUpNoficiationView from the window manager
when it is not currently attached will throw an IllegalArgumentException
so we check if it is attached and if so remove it, otherwise carry on.

As well remove possible left children in statusbarview

Change-Id: Ida916eba496c4183e47288ef24a8a93c46a7eddf