Define VendorApiLevelPropOverride for GRF prop

Soong must read BOARD_API_LEVEL_PROP_OVERRIDE to generate build props
for vendor image. If this value is provided, ro.board.api_level will
be set to this value.

Bug: 374333106
Test: read product_config
Change-Id: Ifbeea76596ea7b22928ae58467bb05cffeb5da1f