Pass back split bounds and meta info with GroupedTaskInfo

* Compute and save a StagedSplitBounds object for every
pair to determine it's position, size, divider, etc.
* Moved GroupTask to Launcher
* Moved StagedSplitBounds from launcher to wm/shell
because sharing is caring.

Bug: 203006080
Change-Id: I8513e027f8c9606fbed1283013eafea2cf905f3f