Support grouping in the New App List

By adding getGroupTitle() to AppListModel.

Default no grouping.

Note: App items should be sorted by group in the getComparator() first,
the new getGroupTitle() will not change the list order.

Bug: 259206313
Test: Unit test
Change-Id: I96fe8615c1f71066288e01d95805a85d5caaff13
3 files changed