String getGroup() {
        return options.optString("androidGroup", null);
    }
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.
To make this notification the summary for its group, also call setGroupSummary(boolean).
A sort order can be specified for group members by using setSortKey(String).

Defaults to null if not set.
src/android/Options.java