gddo-admin: use the new database API for admin commands

Use default context.TODO() when initializing database for usage without
remote_api for search. This CL mainly fixes the gddo-admin commands.

Change-Id: I8518f3aeff57d3e733a99a45c50177cffaf35621
Reviewed-on: https://go-review.googlesource.com/57730
Reviewed-by: Ross Light <light@google.com>