HeadsUp: Correctly add and remove the window

isAttachedToWindow() basically works to determine if the view was attached or not.

But on the boot up process when already addHeadsUpView is called but the system is not
fully up and a recreation happens (eg on TRDS...due that the UIModeManager is doing its work as soon as system is ready)
the view gets attached....but not removed on the recreation due that at this time isAttachedToWindow() will return false even though the
window was added.

To solve simply use a boolean instead of isAttachedToWindow()

Change-Id: Ib576a83cadc581683f2a5b0afc922c53b01888b7

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java