Modify usagestats event processing thread

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

Bug: 303798680
Bug: 302675504
Test: atest CtsUsageStatsTestCases
Test: atest FrameworksCoreTests:android.app.usage.*
Change-Id: I0bdd65d5ef1ed61ab5b8fdf4f1a3a257f1b37111