GitLab

Introduced packager_vars.star so that properties will be the shared
between CI and matching try builders.

Also reorg the following builder to use a similar starlark structure:
* android-androidx-packager
* android-avd-packager
* android-sdk-packager

Bug: 1132368
Change-Id: I8001cfa818c2e647cc513e46a2f166fbe1e253df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625271
Commit-Queue: Haiyang Pan <hypan@google.com>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843125}