Workaround to ensure that PIP activities have a visible shadow.

- When the window for the activity enters PIP, update the outline provider
  to override the alpha of the shadow (to be opaque) to ensure that is is
  visible.  Only applies to the task root activity.

Bug: 36741700
Test: Launch YT, ensure that there is a shadow when after it enters PIP
Test: go/wm-smoke
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: If089dae84e4916d3d0e7bbeb316215b46e522e05