Do not re-use the same EventEntry and DispatchEntry for fallback keys

Re-using the same EventEntry and DispatchEntry for fallbacks is
incorrect. A fallback can be thought of as a completely new event
generated for a channel, in which case it should not share the same
eventId as the original event.

Re-using the same DispatchEntry leads to the incorrect behavior, since
the expected flags in the event will be overridden by the the
resolvedFlags (and other resolved fields) from the old DispatchEntry.
This CL also fixed this incorrect behavior, which is verified with the
new tests added in the following CL.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I30fd72a7da85043ea19fedaace41e79b20c85645