Add function for getting the first vendor API level.

This is the vendor API level supported by the device when it shipped.
The function is needed in VTS tests that exercise functionality
that depends on data provisioned to the device in the factory that
can't be updated when the vendor layer gets updated.

Bug: 384021501
Test: m
Change-Id: I603ba953bb7ccb6bd2518b525bbcdf59af353f34