Switched ActivityStack.mService to ActivityTaskManagerService (7/n)

Objects that contain or represent activities like ActivityStack can
no longer rely on ActivityManagerService as it is going to be in a
different package.

Test: Existing tests pass
Test: go/wm-smoke-auto
Bug: 80414790
Change-Id: I7ea20169245bee91f1be254ac30e88d2382c62a3