ota_from_target_files: read correct api level

ro.product.first_api_level looks like the value we actually want to read
to since it inherits from $PRODUCT_SHIPPING_API_LEVEL. Other variables
we attempted to look at were giving other values (e.g 202504 for pixel
watch)

Bug: 331202590
Test: th
Change-Id: Ied3da247cba45611132db6c9e3cdefb447891f96