Move widget events job scheduling to background thread

The call to JobScheduler.schedule may block for several milliseconds due
to lock contention. Move this call to a background thread so that it
does not block system server start / cause a perf regression.

Bug: 431264524
Test: Boot device with added tracepoints
Test: CtsAppWidgetTestCases:WidgetEventsTest
Flag: EXEMPT bugfix

Change-Id: Id1ff87293a2408f33e2a1479c7c4ea16a0b0d2f7