Avoid double triggering assistant gesture haptic

If the user drags almost all the way to gesture completion and
then flings, the haptic got triggered twice: first for the drag,
and then once the fling was registered. This checks whether the
assistant was already invoked before triggering the fling
invocation.

Bug: 132908798
Test: manual
Change-Id: Ibeed7279b8db32527490a0e11b8e5f0761187bbf