Introduce the product variable PRODUCT_EXPORT_RUNTIME_APIS

PRODUCT_EXPORT_RUNTIME_APIS is a product variable that inherits the
behavior of the build flag RELEASE_EXPORT_RUNTIME_APIS, which is being
removed. This will be utilized in the product `sdk_with_runtime_apis`,
which will be introduced in a future change.

Test: m nothing --no-skip-soong-tests
Bug: 326312949
Change-Id: I4966b7bfc7423db049acfcede67233352ba8341b