Define PRODUCT_SHIPPING_VENDOR_API_LEVEL

Devices may set its first vendor api level to
PRODUCT_SHIPPING_VENDOR_API_LEVEL. This will manually set to
ro.vendor.api_level and will be used by tests.

If this value is not defined, init process will automatically
calculate the ro.vendor.api_level property for tests.

Bug: 326164316
Test: Check vendor/build.prop
Change-Id: I89f844ffaad9e3e39deab1c680bd1f7f17f4d8ad