Fix GroupMembershipManager implementation again.

We decided to keep this class answering questions related to the
attachment state of the notification - so if a notification is not
attached, it makes sense that it would return null/false for most
things. In this case, we need to update the callsite if we have to do
something with entries that are not attached.

Bug: 299255526
Fix: 293434635
Test: GroupMembershipManagerTest + GroupExpansionManagerTest +
tested manually that single notifications and groups can be expanded +
tested the expansion behavior after dnd on/off +
released the flag in another CL to run scenario tests

Change-Id: I5f015d9d3e07de16b9c40af2589929296d96814b