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
(cherry picked from commit 6a7eb48b34ef03d7eafa88921a51b2c364b1baee)
Merge pull request #48 from CyanogenMod/stable/cm-11.0

HeadsUp: Only remove heads up if attached to window