Persist UsageStats before backup.

This fixes a bug where in-memory usage stats were not being backed up,
potentially causing a loss of up to 20 minutes of data when restoring.

Fixes: 155930164
Test: atest UsageStatsDatabaseTest
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: I454f64e7de3f7a6046c408859998a3e51d5d338d