Add flex split flag for resizing for launch adjacent

* It accidentally resizes on non-launch adjacent use cases,
see bug for more info

Bug: 383624769
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: No resizing happens when flag is off
Change-Id: Id22ee84d77ba6c89bd75e88fbe53bf10c8d16fdb