Ensure view is attached to window before removing it

I'm not able to reproduce the bug, but we can avoid calling removeView
if the view isn't attached.

Bug: 180114844
Test: Manual verification of existing behavior
Change-Id: I1173f7d3656aaa10815f07717c09d3a3869d9878

packages/SystemUI/src/com/android/systemui/assist/AssistManager.java

@@ -170,7 +170,9 @@ public class AssistManager {
                     boolean visible = false;
                     if (mView != null) {
                         visible = mView.isShowing();
-                        mWindowManager.removeView(mView);
+                        if (mView.isAttachedToWindow()) {
+                            mWindowManager.removeView(mView);
+                        }
                     }
 
                     mView = (AssistOrbContainer) LayoutInflater.from(mContext).inflate(