Rename to getNotificationChannelGroupsWithoutChannels

This change renames getNotificationChannelGroups() in both RankingConfig and PreferencesHelper (the only implementer of the interface). There are other methods in PreferencesHelper that are already called getNotificationChannelGroups(), so this rename clarifies the difference for this one, which gets just the group objects.

Flag: EXEMPT rename
Bug: 381131846
Test: PreferencesHelperTest
Change-Id: Ib496e9add1a199814dc45f1a099c6c20487f4955