Modify usagestats event processing thread

Use a dedicated thread with default priority to handle
the usage stats event processing.

Also offload IOs to the dedicated IoThread and post processing
the launch time estimation.

Bug: 302675504
Test: atest CtsUsageStatsTestCases
Test: atest FrameworksCoreTests:android.app.usage.*
Change-Id: Ib0bb490279c1dcbd71cabfc3a1f7d97c638eb9b6