Allow TaskDisplayArea to have TaskDisplayArea children (1/n)

- Remove TaskDisplayArea#getRootTaskAt(int index) because the child at
the requested index may not be a Task.
- Change root task traversal to use forAllRootTasks().

For usages in TaskDisplayArea, update to traverse using mChildren for
now. Will revisit them when changing the signature.

Bug: 175136051
Test: manual for refacotor
Change-Id: I36cd6edc99c3218743cde5e64c7892e91cde63b5