Refactor methods for returning groups with channel lists.

This brings the logic for filling in the channels associated with a group given the list of channels and options for which channels and groups to include. This refactor is in preparation for caching group info on the client side.

Bug: 381131846
Flag: EXEMPT refactor
Test: PreferencesHelperTest, NotificationChannelGroupsHelperTest
Change-Id: I95d27e3c3ded0204cf41e1b8e1f74adc7396850b