switch to libstdc++ for standalone Android builds.

This makes it possible to target NDK API 18 (K) again.

Change-Id: Id3d1f19b2904792b4001d2ea0942cc1ab6cf732e
Reviewed-on: https://skia-review.googlesource.com/6081
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed