Update how UsageStats timestamps are written to disk.

When writing UsageStats timestamps to disk in proto, we offset the time
by the begin time to save storage space. However, there was logic which
reset default timestamp values of 0 to begin time for those stats.

This CL standardizes how timestamps are written to disk in proto for
UsageStats and updates the parsers. Since backup and restore use
database version 4, UsageStatsProto is also updated in addition to
UsageStatsProtoV2.

Bug: 155149300
Test: atest UsageStatsDatabaseTest
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: I624e4d56c91762b56eb1c3be67cd8df033fcac95