Update a11y strings for flexible split

* Temporary solution, long term and hopefully more scalable is
to try to re-land ag/33667759

Test: Manual
Bug: 415827083
Flag: com.android.wm.shell.enable_flexible_two_app_split
Change-Id: I7beaeee40a81efcfcb2b0ebb3c91c9bf43129fde