Update task descriptions when reparenting or reordering children

- The task description coalesces information from the descriptions
  provided by the child task's activities, so when the children or
  their ordering changes, we also need to update ancestor task's
  task descriptions as well.

Bug: 339274381
Test: atest TaskTests
Test: Revert ag/27365753 and try the steps in the bug
Flag: EXEMPT bugfix

Change-Id: I3d859cad9b036c823306bd7122988e7b23ac07f7