InputDispatcher_test: Use InputEvent::nextId() for all injected events

Input events are tracked through the pipeline using their event ID.
There is an underlying assumption that each new event will have a
different ID (apart from the small chance that there's a collission in
the randomly-generated IDs). This assumption needs to be fulfilled for
features like tracing and latency tracking to work as intended.

Before adding tests to ensure all events are traced correctly in
tests for InputDispatcher, we must uphold this assumption and use a new
eventId for each injected event.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: Icbbd3eb6d62f9de1c9320207a4d6fa6070f2b30b