Remove obsolete Android versions from packager.

Remove packages for SDK 28 and earlier as we've been using SDK 29 for
some time already. Keep SDK 29 for now until the SDK 30 roll is
successfully completed.

Change-Id: Ib15bd1330c95835e61df416192620048c45e5a22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305011
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789629}
GitOrigin-RevId: 2ad17d4e1bf3f4012fdc39fd1866ec1f3a7c3a61