Add TestApexes to ApexInfo

If any of apexes in apex_available is an apex_test, then that name will
be propagated down from that apex to each apex variant. This metadata
will be used to enforce that stub libraries cannot have more than one
apex_available.

This logic is necessary so that bp2build can select the correct
stub/impl.

(To avoid replicating this complexity in Bazel, we should consider
dropping the test apexes in Bazel BUILD files, next CL)

Bug: 277651159
Test: go build ./apex
Change-Id: I63617c1dc2a2d5c9cd7758c416fec7b4db1f10a7