Fix the invalid value of Elapsed property

The Elapsed property is described in microseconds in the interface but the result value is in seconds before

Tested:
1. get the property by this: busctl introspect xyz.openbmc_project.Dump.Manager /xyz/openbmc_project/dump/bm:c/entry/20 xyz.openbmc_project.Time.EpochTime
2. check the value
NAME                               TYPE      SIGNATURE RESULT/VALUE     FLAGS
.Elapsed                           property  t         1645758497000000 emits-change writable

Change-Id: Ib6cd5261c4cd6ac7de239f291a9f5fb556b29e89
Signed-off-by: Xie Ning <xiening.xll@bytedance.com>