Allow split pair to move to external display

This CL allows split pair to move from primary display to external
display, when the flags are enabled.

Bug: 392967255
Test: manual
Flag: com.android.window.flags.enable_non_default_display_split
Recall: http://recall/clips/c1312d26-a9ee-4f10-9ff7-1fc55376365b
Change-Id: Ida41fd34e4522d703b78d17312feded2edeb440d