bp2build: Fix apex.binaries to be a LabelListAttribute.

apex.binaries accepts a list of module names, which should be resolved to
their fully qualified labels. Bazel treats string_list and label_list
attributes differently, most notably with the latter adding dependency edges.

Test: apex_conversion_test.go
Bug: 209743852
Change-Id: Iafdc5c728e8658cce0e99d42f32e7bb6fe2f3168