InputTracer: Ensure eventProcessingComplete called after dropping events

There should be no more dispatching decisions made after notifying the
tracer that event processing is complete. However, there was a codepath
where we were previously dropping events and synthesizing cancelations
after calling eventProcessingComplete().

Move the eventProcessingComplete() call to after handling dropped
events.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I4b61ef85f6caef345171bfd0a5a01c4367573512