Define BOARD_API_LEVEL and BOARD_API_LEVEL_FROZEN

BOARD_API_LEVEL and BOARD_API_LEVEL_FROZEN are set by the release
flags.
BOARD_API_LEVEL sets ro.board.api_level that shows the API level of
the vendor API surface.
BOARD_API_LEVEL_FROZEN sets ro.board.api_frozen that shows if the
ro.board.api_level is finalized.

Bug: 295269182
Test: getprop ro.board.api_level
Change-Id: Ie57c57b6c9f1fc0c98968195843059a48da8e512