InputTracer: Wait to trace dispatch events to the backend

... until event processing is complete. This is because we need to wait
until processing complete to get information about the targets and
sensitivity of an event, which is required to write the dispatch event
to the backend.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I227c207d6a5667f9297e74b4c8be9f8e980d6d13