Add flag AMOTION_EVENT_FLAG_IS_GENERATED_GESTURE to Input.h

Since there is no support for touchpad gestures yet, the gestures are
sent as MotionEvents with the flag:
AMOTION_EVENT_FLAG_IS_GENERATED_GESTURE

This flag is already present in java as:
MotionEvent.FLAG_IS_GENERATED_GESTURE

This change adds the flag to Input.h.

Bug: 119265089
Test: none
Change-Id: Ide0cc41d3f44a8694fa02faa0337eb2cf72de6f2