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.

Change-Id: Ida916eba496c4183e47288ef24a8a93c46a7eddf
Merge pull request #4 from CyanogenMod/cm-11.0

HeadsUp: Only remove heads up if attached to window