image_signing: unit tests for Android image

- move helper functions that detect which keys should be used depending
on the build flavor to a separate lib
- add unit tests for that lib

BUG=b:72947583
TEST=unit tests
TEST=run against caroline image, scripts detects 'cheets' build flavor
TEST=run against novato-arc64 image (SDK), script detects 'cheets' build
flavor
TEST=run against newbie image (AOSP), script detects 'aosp' build flavor
TEST=run against invalid build property 'paosp_cheets_...', script
aborts as expected
BRANCH=None

Change-Id: I5595c10a5a063e7658d0cf17c77dbeead429cd97
Reviewed-on: https://chromium-review.googlesource.com/923097
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed