Offload some processing to IoThread

Use dedicated IoThread for bg/fg proc status updates and launch
time prediction.
Avoid holding the main lock for launch time prediction

Bug: 302675504
Test: atest CtsUsageStatsTestCases
Test: atest FrameworksCoreTests:android.app.usage.*
Change-Id: I88f8d3d160db6404282ea73374e6a2c8d05f05cf