Fixed an issue where notification groups could be empty

Because we were looking at the theoretical and not the actual
showing public state, notifications could get updated to
invisible without ever recovering.
The actual showingPublic state now determins the visibility.

Fixes: 117272628
Test: add low priority child while group is on lockscreen and invisible
Change-Id: Iacdd984451a2e3d69a0aaa8186b6afab5447188e