Replace manual creation of StatsEvent objects with their appropriate buildStatsEvent() method

Currently, Java clients are manually creating the StatsEvent objects
which can lead to errors. Now that there is support for pulled atoms for
stats-log-api-gen for Java clients, use the appropriate buildStatsEvent
method that was auto generated by stats-log-api-gen.

Bug: 160368804
Test: Ran `m` and completed successfully

Change-Id: I086ef7dd07a65e00658fc4170d0b20b08764ceaf