Disable touch slop for generated gesture events

This accounts for a special flag for gesture events generated by
InputFlinger. For these events we do not need a touch slop since we
can reliably tell that the intention is to scroll.

Bug: 31799135
Bug: 32944705
Test: None
Change-Id: Ibca8eaebca61bd4eb401708f5b19463c19c23309
(cherry picked from commit 8e07f0f91d8e6e8965faaaadc94b4bac18ba3310)
(cherry picked from commit 3e1b4218e6f0117e58a5452bcb725b105d32b528)