Simplify adding FLAG_IS_GENERATED_GESTURE in GestureConverter

This ensures that IS_GENERATED_GESTURE flag is always added when the
gesture is two fingers scroll.

With this change, FLAG_IS_GENERATED_GESTURE is added to fling stops.

Bug: 364460018
Test: GestureConverterTest
Flag: NONE very minor behaviour change
Change-Id: If18f65395e3a2d3eabf514a1fe8660de2a165978