Fix the VSR_VENDOR_API_LEVEL calculation

VSR_VENDOR_API_LEVEL is used to know the vendor API level for the VSR
requirement.
This must have the same value with the ro.vendor.api_level on run
time.

Bug: 332401254
Test: get_build_var VSR_VENDOR_API_LEVEL
Change-Id: I8e1fac8f05e4b06128738f377d63f26567d3902b