Removed mStackId (86/n)

mStackId is the same as mTaskId since all stacks are now tasks
Also, switch some of the users to use getRootTask()/getRootTaskId()
where it makes sense.

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ie625bf8734ca01646f6f45705edf0eb240819c24
20 files changed