Upgrade android build (and infrastructure) to use ndk r15.

Bug: 6672
Change-Id: Ia6c6bae8a9adfd75172d7116487dfa63c3d69670
Reviewed-on: https://skia-review.googlesource.com/20904
Commit-Queue: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Eric Boren <borenet@google.com>
8 files changed