Roll NDK to r12b and extract it into its own repository.

BUG=599327

Change-Id: Ife24c04e47ccfc41e58562d3e27bbcef2014e831
Reviewed-on: https://chromium-review.googlesource.com/380499
Tested-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>