Restrict usage of PopupWindow::mOverlayAnchor

The field mOverlayAnchor has getter and setter since API 23 and it's a
recommended way to access and set this field. All the usages from
opensource is mostly AppCompatPopupWindow forks (some with version
check, some are not), so they can easily migrate to getter and setters.

Bug: 123768692
Test: PopupWindow and PopupMenu tests passed
Change-Id: Ida3077b17a043b2acf2d89456e4501d7ed052e4e