Multi-tree API bp2build converter for ndk_library

This is a partial bp2build conversion that only emits the
cc_api_contribution target of ndk_library. We also need to convert this
to cc_stub_suite eventually to enable building vendor/product API
domains with Bazel

Also create an enum for the three known Multi-tree API surfaces (will
likely be expanded in the future)

Test: go test ./bp2build
Test: b cquery //bionic/libc:libc.ndk.contribution --output=starlark
--starlark:expr="providers(target)"

Change-Id: Idb24871ba20aae132b61eb31ef35c917cacae9e1