See go/refactor-group-task for details. This CL removes all the usages
of `task1` and `task2` from `GroupTask` in Launcher3. Follow-up CLs will
remove it from NexusLauncher and remove the fields altogether.
Bug: 388593902
Test: m
Flag: EXEMPT pure refactor with no behavior change.
Change-Id: I902c8135b3a0aae95acf25267b3bcbf286bd4e7d