Remove mTargetViewContainer safely

Per bugreport there is chance that
PipDismissTargetHandler#mTargetViewContainer is not attached to window
manager when we try to remove it in PipDismissTargetHandler#init on
overlay changes (such as changing wallpaper)

Bug: 209796452
Test: N/A not able to reproduce on raven
Change-Id: Ic2fbe2d59145afa2c994bc75446d60f8c4415e58