Add notification resurface delay flag

Defines a new duration-type flag to control how long SC must wait
before resufacing notifications which are dismissed.

By default (with duration set to zero) the notifications never resurface.

Implementation (usage of this flag) and tests to follow.

Bug: 266680614
Test: N/A
Change-Id: I8f3b235f789c2bf7613dc97fec1642c3ab986616