Add a script and documentation for building the Viewer APK.

BUG=skia:6009

DOCS_PREVIEW= https://skia.org/?cl=6660

Change-Id: Ibcf0d1e7f3519e703bc74f0329d0ac92110e70ec
Reviewed-on: https://skia-review.googlesource.com/6660
Commit-Queue: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
3 files changed