fix: inconsistent json and yaml spanner configs (#238)

* fix: inconsistent json and yaml spanner configs

This tries to resolve inconsistent timeout/retries in different client
libraries.

Two major changes:
- Update *_grpc_service_config.json and *_gapic.yaml to make sure that they
  are consistent.
- Add missing ListDatabases config in spanner_admin_database_gapic.yaml.

PiperOrigin-RevId: 312582296

Source-Author: Google APIs <noreply@google.com>
Source-Date: Wed May 20 16:55:15 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: d8a17933f650cf605196ef14f03edc246f0562b6
Source-Link: googleapis/googleapis@d8a1793