This adds an ndk_next gclient configuration to allow bots to use the
upcoming version of the Android NDK.

BUG=668202

Change-Id: I403e398094f4215cdb3783b68f465323264ca1ee
Reviewed-on: https://chromium-review.googlesource.com/414184
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Mike Stipicevic <stip@chromium.org>