Renamed some getXXXStack methods to getRootXXXTask (87/n)

Also stacks are tasks now so this is "techincally" correct.
Also, stacks are going to be removed soon, so this CL helps reduce
size of future CLs.

Bug: 80414790
Test: Existing tests pass.
Change-Id: Ibc47b63f19fd154b9162264f0ad1c9259a3e5d56
20 files changed