vts: kernel: toolchain: prefer ro.vendor.api_level over ro.product.first_api_level

ro.board.first_api_level
seems to be the preferred property for devices launching at S but
with vendor implementation frozen at R, according to:
https://docs.partner.android.com/gms/building/integrating/extending-os-upgrade-support-windows#new-system-properties
https://docs.partner.android.com/gms/building/integrating/extending-os-upgrade-support-windows#example_grf_chipset_3
but it may not be defined for non-GRF chipsets. Prefer
ro.vendor.api_level.

Bug: 210928656
Test: mm && adb sync data && \
      adb shell /data/nativetest/vts_kernel_toolchain/vts_kernel_toolchain
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I7e9ef19b8349eca77315b6c7aa89eea01e308ab3
1 file changed