Add distance threshold for assistant gesture fling

It was possible to invoke the assistant accidentally. This change
adds a minimum distance threshold before we register a fling (the
same distance as used for drag, 55dp).

Bug: 137106918
Test: manual; tested that accidental flings were much more difficult,
but intentional invocations were still easy to register

Change-Id: I40c8bd43c1a28c7b161467804a1e44746b8e92ef