Allow NDK APIs to be marked as drafts.

Draft APIs are available to the platform and to CTS to allow
developers to iterate on an API, but hidden from the NDK artifacts to
avoid releasing the API until it is ready.

Test: Mark binder_ndk headers and library as drafts, make checkbuild,
      build-ndk-prebuilts.sh, verify missing from NDK artifact.
Bug: http://b/120091134
Change-Id: I8685e92bdaaea581e17fe98e7a2bfb9388f9f132