Rename NotificationEntry.isChildInGroup()

The old NotificationEntry.isChildInGroup() currently represents whether
the corresponding notification row is a child in a group, not the
NotificationEntry. The grouping status of a NotificationEntry is not
always equal to its row's, depending on the stage of the Notification
Pipeline. We want the name NotificationEntry.isChildInGroup() to
represent the NotificationEntry's grouping status for the usage of async
inflation and other pipeline jobs.

Bug: 217799515
Test: presubmit
Change-Id: Iac2186d58671c9b53c2996f7b313fc4634e8a0ad