Add variable PRODUCT_SHIPPING_API_LEVEL and make files with the

new read only product property ro.product.first_api_level.

PRODUCT_SHIPPING_API_LEVEL declares which API level a device first
shipped with. We use this variable to set a read-only property that
can be used in CTS/GTS tests.

Change-Id: If54202075d3b92ed380655cde8b3fc50c5ba73b3