NoMan: Optimize grouped notifications

When SysUI is the only notification listener, drop group children
if there is a group summary in NoMan.

There are two changes needed to achieve this:
1. Cancel children when a summary is posted
2. Drop children if there is a summary

Bug: 18460939
Change-Id: I5a3f30b0b23b2a784783749f58352c05a7fb9e59