Use board api level for seapp coredomain check

Rather than PRODUCT_SHIPPING_API_LEVEL, use board api level
(BOARD_API_LEVEL or BOARD_SHIPPING_API_LEVEL) to determine whether we
check coredomain violations or not.

Also provides a Makefile variable to override the flag, for targets that
want to turn on the check optionally.

Bug: 280547417
Test: see build command of vendor_seapp_contexts
Change-Id: I177630d33313334ca4a56a9be88b78cff678281e