Fixing regression in PiP TV.

- Don't clobber the state if we are deferring resizing due to finishing
  activities.
- Fix issue with PiP tasks being visible after the stack is removed, due
  to it being put below a non-fullscreen task in the fullscreen stack.
  Instead just move it to the back of the stack.

Bug: 36592307
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: Ia18fe155b8a045a8ffea84612fd08af3ef3982d8