Add test case for hover eventId synthesis

When a new event is synthesized in InputDispatcher causing its action to
change, the synthesized event should always have a new eventId. Verify
this by adding a test case where hover enters and exits are synthesized
from hover moves, and ensure the synthesized events have a new eventId,
while move events retain their original id.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: Icb413f412dd68a0e6bfb056af18ef5e0935facef