InputTracer: Ensure tracer thread is destructed first

InputThread stops when its destructor is called. Initialize it
last in ThreadedBackend so that it is the first thing to be
destructed. This will guarantee the thread will not access other
members that have already been destructed.

Bug: 330211703
Change-Id: Ia50585d8276fe87d40d5528b3eb9480a64ea9854
Test: None