public Builder setGroup(String groupKey) {
            mGroupKey = groupKey;
            return this;
        }
Set this notification to be part of a group of notifications sharing the same key.
Grouped notifications may display in a cluster or stack on devices which
support such rendering.

<p>To make this notification the summary for its group, also call
{@link #setGroupSummary}. A sort order can be specified for group members by using
{@link #setSortKey}.

@param groupKey The group key of the group.
@return this object for method chaining
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/compat/NotificationCompat.java