Postpone headless app caching to after boot.

Moving the headless app cache loading to the background thread still
affects boot time. Moving the loading to after boot has completed means
that it will no longer impact boot time.

Bug: 159057406
Bug: 159434503
Test: atest CtsUsageStatsTestCases:UsageStatsTest
Test: atest FrameworksServicesTests:AppIdleHistoryTests
Test: atest FrameworksServicesTests:AppStandbyControllerTests
Test: Confirm SystemServerTiming_OnBootPhase_500_com.android.server.BinderCallsStatsService$LifeCycle timing decreases
Change-Id: Ib79939ed15f9886d4fea02f7eda172b8cc82b674