Define ro.board.first_api_level property

When a device define BOARD_SHIPPING_API_LEVEL with an API level, it
sets a vendor property ro.board.first_api_level in vendor/build.prop.
This is for the GRF devices. Non-GRF devices must not define this
property.

Bug: 176950752
Test: getprop ro.board.first_api_level
Change-Id: I6921d7fe6acca3f73a5fd0fbaa3d4f1e9394541b