Frameworks: Be sure heads up view is attached before removing it

We did not have it in our tests but some user ran into it. Thing is if the
system is disabling notifications for whatever reason and till this time the
view was never attached and the system tries to detach the view on windowmanager
we run into an illegal argument exception.

Fix it with simply checking if the view was attached before removing it.

Change-Id: I0800beed3553815cc529087052138345773b32e4
Merge "Frameworks: Be sure heads up view is attached before removing it" into lp5.0-caf