[task scheduler] Use DB backup file creation time
[task scheduler] Use DB backup file creation time

In GCS, the file update time is changed by the process that
automatically changes the storage class, which causes the DB backup
metrics to be useless.

Change-Id: Iec7641dbd1862fae9ad638d9b79900c4657d272b
Reviewed-on: https://skia-review.googlesource.com/c/191240
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

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