Renaming Stacks APIs to RootTasks

This CL is aimed to refine the AM/ATM API surface. The other
stack naming internally spread in AM and ATM will be covered
in later CLs.

Also Consolidate #removeStack() with #removeTask().

Bug: 157876448
Test: existing tests

Change-Id: I1254f972dab3bbe66a736a0402eebce9d0fb1496