Finally remove `task1` and `task2` from `GroupTask`

See go/refactor-group-task for details. This is the final CL
in this refactor, where `task1` and `task2` are finally removed,
and `DesktopTask` is allowed to have empty tasks.

Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: I8af39b58138f11030981311efc2e95a77cef125b