Allow TaskDisplayArea to have TaskDisplayArea children (2/n)

- Small clean up of several usages of
  forAllTaskDisplayArea(tda -> forAllRootTasks(...)) to forAllRootTasks.

Bug: 175136051
Test: manual for refacotor
Change-Id: Icd67b807cc71d3aa1d693d4fc5ba91a2cf14bae3
3 files changed