Duplicate the last_install content into last_log.

Currently we save the OTA metrics in last_install, which keeps the data
for the _last_ install only. This CL logs the same content into last_log
so that we keep the metrics for every install.

Bug: 31607469
Test: Apply an update (via OTA and sideload) and check last_log and last_install.

Change-Id: Id8f174d79534fddc9f06d72a4e69b2b1d8ab186c