Temporary affordance for blocking helper

When a notification is flagged by NoMan as USER_SENTIMENT_NEGATIVE (odd,
since this is actually the system's sentiment), a small ⛔ icon will
appear that directs you to the notification settings.

Eventually the icon will be removed, and the settings (reworded to
explain that you seem not to like this kind of notification) will be
left behind in the shade.

Bug: 63095540
Test: runtest systemui
Change-Id: I8b815cd035e9730bbbf1d4a1be17db9d494111ed