Define ro.vendor.api_level

ro.vendor.api_level will have the api_level that the vendor images
are required to implement based on the various api level properties.
ro.board.api_level and ro.board.first_api_level explicitly states
the api level of the vendor images for grf.
If these properties are not provided, it reads the device's launching
api level.
The rest of the properties may show the implemented api level of the
vendor images.

Bug: 200258600
Test: getprop ro.vendor.api_level
Change-Id: Id4131b53d6db00d5bbef3b048384d33c1736d681