UsageStats: Don't trigger idle check for USER_SYSTEM during boot up

It may trigger wrong or unnecessary bucket evaluation/reporting
during the boot phase, a full idle check will be triggered after the boot completes.

Bug: 337864590
Test:atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Change-Id: I6415d66b7706fd3a04164cdba1f2e0981345d8b4