Ensure consistent stack behind PIP when going to/from fullscreen.

- When a fullscreen activity enters PIP, we were previously always
  bringing the home stack forward due to the return type being set
  to HOME_ACTIVITY_TYPE, causing the home screen to be visible if
  expanding PIP over a fullscreen activity and going back into PIP
  immediately.  This CL instead updates the return-to type to
  reflect the stack that was currently open before the pinned task
  was made fullscreen again to ensure that it is consistent when
  entering PIP again.

Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testPipUnPipOverHome
Test: #testPipUnPipOverApp

Change-Id: Id5dd7db1f5a8040ff21190cc347eb6aae5893006