InputTracer: Disallow adding new targets from a derived event tracker

There are complex interactions between features in InputDispatcher that
can lead to cancelations being generated for a derived event. When
this happens, calling dispatchToTargetHint will result in a crash.

Since it is difficult to determine exactly how to reproduce this issue
in a test case, loosen the check so that we only crash if we are trying
to add a new target, which should never happen.

Bug: 328928072
Bug: 210460522
Test: will re-enable the flag and wait for new crash reports
Change-Id: I0134a12b40e1da33a94d61b721e967dfdbc95fe8