Use (the last existing timestamp in DB + 1) as the start time of usage

event query to avoid loading the same events repeatedly.

queryEvents API will return the events within [start, end). In the
hourly job, if we query the events from the latest existing time, the
events happening at the latest time will be saved for multiple times
into database. This aims to avoid this issue.

Test: make RunSettingsRoboTests
Bug: 265110147
Fix: 265110147
Change-Id: I408e88b0e15fe22585906261935854cf47707f9c