Flush Content Capture context updated event immediately

Since setLocusContext() usually manually set/reset by developer to update
the app state, it makes sense to trigger a flushing to deliver the
update events immediately.

Bug: 191169236
Test: manual, check the events flush immediately
Test: atest CtsContentCaptureServiceTestCases
Change-Id: I0c367ec083b96d60600ca375c2d70c53224e9b62