Add Android API 26 (Oreo) system image to android-sdk-packager

We are not adding google_apis_playstore image as the tests in
chrome_public_test_apk don't seem to require playstore now. See
https://crbug.com/1434504#c6 for an example.

Bug: 1459433
Change-Id: I526be01171656587aef0924a1871435af8d14086
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4678591
Auto-Submit: Haiyang Pan <hypan@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1169706}
NOKEYCHECK=True
GitOrigin-RevId: 40e9592018e4ec206fbe031739c81ee7ab0176a6