Add Android S preview system images to android-sdk-packager.

Since Android S preview is available for public (aka in sdkmanager),
add the system images so that users can use them for emulator.

Bug: 1221261
Change-Id: I719a602d96a57f85bcdc11e5608aa2f95c4b9494
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2971103
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#893553}
NOKEYCHECK=True
GitOrigin-RevId: f74a93e1567af5efae46b21d5e1479b8e05f1910