Replace ndk_libs.bzl with an attr in cc_stub_suite

(This CL is a cleanup, and should be a no-op)

Currently we support three variations of cc api stubs.
1. publicapi stubs, i.e. ndk stubs (empty additional args to ndkstubgen)
2. module-libapi stubs that are also an ndk library (--systemapi --apex)
3. module-libapi stubs that are not an ndk library (--systemapi --apex
   --no-ndk)

ndk_libs.bzl was used to differentiate between (2) and (3). This creates
an additional layer of indirection - users will need to modify this
external .bzl file if they would like to add a library to an ndk.

Replace this with an explicit atttibute in cc_stub_suite macro for better UX.

Test: go test ./bp2build
Test: b test //build/bazel/rules/cc:cc_stub_library_tests (added in
sibling CL)
Bug: 299501496

Change-Id: Idd3579e8013bae7a1740534f90d2767df5bac1a5