Set BOARD_SYSTEMSDK_VERSIONS even if PRODUCT_SHIPPING_API_LEVEL is unset

Previously, we checked if PRODUCT_SHIPPING_API_LEVEL >= 28, but that no
longer is needed because upgrading from 29 to 35 is not expected. All
devices running Android V are expected to be shipped after 28.

Furthermore, this fixes non-device targets (like sdk) where
PRODUCT_SHIPPING_API_LEVEL is unset.

Bug: 318634417
Test: build with sdk target
Change-Id: I8f8337fb76312334777e621ae5870d953167ad49