Better deletions

- Allow apps to delete notification groups (which cascade to delete all
channels therein)
- Allow apps to get their groups, so they know what they might want to
delete
- Don't throw if someone tries to delete something that already doesn't
exist.

Change-Id: I5f6e25497c5da1e57b52737586e86097332f88f9
Fixes: 36245468
Fixes: 36127382
Test: runtest systemui-notification, cts