Prevent setting task thumbnail during swipe to overview

Previously we were setting the incorrect thumbnail when
swiping to overview, then taking a task screenshot right
when the animation ends and replacing the first thumbnail.
This sometimes caused a flicker of the old thumbnail, now
we avoid setting the thumbnail at all for the taskview that
is being swiped up.

Another edge case handled here is when switching nav modes
and then immediately entering overview, Recents receives an
onConfigChange for changing task icon size. That would cause
all taskviews to null out both their existing icon and
thumbnail, now only the icon gets nulled out.
Existing issue where switching to 3 button nav and then
entering overview shows blank icon, doesn't register for
receiving the updated task snapshot fast enough.

Fixes: 179307265
Test: Ask assistant for weather, swipe to overview
Ask assistant for time, swipe to overview, no flicker

Switch nav modes to gesture, swipe to overview.
There's a flicker for config change, but thumbnail is correct

Change-Id: I300b29f999f6d6876f82bc0189b44f4c10ae33fe