Fixing bugs in Notification proto dumping.

Also changing method signatures to be more in line with our convention.
Based on some of the changes I went through, it looks like:
1. We were only legitimately saving the very last NotificationRecord
that was dumped.
2. We weren't dumping any NotificationChannels into
NotificationChannelGroup protos.

Bug: 65750824
Test: Android builds
Change-Id: I8c1ef90cf69d8ea00a7bad0c67445741056e23ef