apex_key bp2build: disambiguate module and src deps.

(and remove unused keyName field.)

The private_key and public_key props of an apex_key can point to either
a module or a string. If it's a module, then respect it. If it's a
string, there's additional product variable lookup to find the apex_key
files in product_vars's DefaultAppCertificate parent dir.

This is similar to android_app_certificate.

Test: presubmits
Change-Id: Ib258da14cb0c2df8b5f817fcbc46afebcf225db8