Ensure at most one pip-dismiss-overlay

PipDismissTargetHandler#init can be called multiple times, whenever a
new dismiss view (and its container) is created, remove the old one from
view hierarchy first.

Bug: 207495181
Test: manual, change wallpaper multiple times to trigger multiple init
      calls in PipDismissTargetHandler and ensures there is at most one
      pip-dismiss-overlay in the view hierarchy.
Change-Id: I4a0e058d1d002a6420974f41b94b70316d3444a3