Introduce TopTaskTracker.getPlaceholderGroupedTaskInfo

This CL just introduces an API inside TopTaskTracker to return
a GroupedTaskInfo, no existing logic being migrated to it yet.

It will be used to replace `getPlaceholderTasks` and
`getSplitPlaceholderTasks` in the follow-up cls.

Bug: 401582344
Flag: EXEMPT, introduce a new API without usage
Test: m
Change-Id: Ic12003833605582d191c4abfd4a0878c266dd30f
2 files changed