Make `GroupedTaskInfo.forSplitTasks()` take non-null SplitBounds

See go/refactor-group-task for details.
ag/31398319 introduced `SingleTask` and `SplitTask` as subclasses of
`GroupTask`, and enforced that `SplitTask` accepts only non-null
`SplitBounds`. This is the Shell-side CL of this change.

Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: Ib9aafb641ceada40f9097d7d39d11334f38b8c82