InputTracer: Use explicit thread wake conditions

This is a small cleanup to use the stop_predicate for the
std::condition_variable::wait() method to make the waking conditions
explicit and more readable.

Also, move the events buffers out of the thread loop to avoid the need
to reallocate it for each iteration.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: Ic56a6f2cf42fe2eee752e1d34fd347a3f5421991