Report three- and four-finger swipes

The dispatcher still needs to be modified to only dispatch these to
SysUI windows.

Bug: 251196347
Test: check events received by a custom tester app, and touches shown by
      pointer location overlay
Test: atest inputflinger_tests
Change-Id: I3a7211d4a67e6388231bef158d3748c2e72e128d
diff --git a/include/android/input.h b/include/android/input.h
index 5d19c5c..a0b46de 100644
--- a/include/android/input.h
+++ b/include/android/input.h
@@ -870,6 +870,14 @@
      * The current event stream represents the user swiping with two fingers on a touchpad.
      */
     AMOTION_EVENT_CLASSIFICATION_TWO_FINGER_SWIPE = 3,
+    /**
+     * Classification constant: multi-finger swipe.
+     *
+     * The current event stream represents the user swiping with three or more fingers on a
+     * touchpad. Unlike two-finger swipes, these are only to be handled by the system UI, which is
+     * why they have a separate constant from two-finger swipes.
+     */
+    AMOTION_EVENT_CLASSIFICATION_MULTI_FINGER_SWIPE = 4,
 };
 
 /**