Use current display as parent when exiting split screen

When split pair is dismissed in connected display, the stage tasks should stay in current display area. This CL updates the stage task's new parent as current display token.

Test: manual
Flag: com.android.window.flags.enable_non_default_display_split
Bug: 437145187
Bug: 438324415
Change-Id: Ib0c569b6a39da513688e6ead4f86352e820c54e7