Fix z-order issue with text cursors and PiP.

- When an activity is in PiP, its windows are top most and always
  computed incorrectly as the IME target. As a result, the popup
  windows for the cursors don't find the right parent and are not
  promoted to be above the IME.

Bug: 63546389
Test: Enter PiP, open app with bottom anchored input, and select
      some text.  Ensure that the cursors are top-most.
Test: go/wm-smoke

Change-Id: I1267504e10544b0a075bbf73e41e001d07a5f74c