Add IS_GENERATED_GESTURE flag to touchpad scrolling

The IS_GENERATED_GESTURE flag was added to improve scrolling in ARC++,
and tells GestureDetector that the finger going down and moving is
definitely for scrolling, meaning that it doesn't wait until ~20 pixels
of movement have happened before starting the scroll. Adding it here
should make scrolling a little more responsive, and also make the events
more consistent with those for scrolling on ChromeOS.

Bug: 251196347
Test: atest inputflinger_test
Test: check touchpad scrolling still works well
Change-Id: I17ac063fdea1f38413fd8060258c79bff3ccd5c0