Moved some activities implementation to ActivityTaskManagerService (2/n)

Second step in unifying the window hierarchy that is currently split
within AM and WM packages. We move some of the API implementation for
activities from ActivityManagerService.java to
ActivityTaskManagerService.java.

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

Change-Id: I23dcd924493d8ad1e0b6e3a55386fd72b0146605