InputVerifier: only check pointer sources

Check for the source inside InputVerifier. Sources like MOUSE_RELATIVE
could send ACTION_MOVE events without a prior DOWN event. Verifying such
streams is tricky, so let's simply skip such events for now.

Also in this CL, add some verifications to the number of pointers inside
the event.

Bug: 211379801
Test: enable event verification and run native tests
Test: atest inputflinger_tests libinput_tests
Change-Id: I3703ba57af7ede77712b91b7429ac46c0624a616