HOME_STACK_ID, ASSISTANT_STACK_ID, and RECENTS_STACK_ID be gone!

Replaced usages with ACTIVITY_TYPE_HOME, ACTIVITY_TYPE_ASSISTANT
and ACTIVITY_TYPE_RECENTS.

Test: go/wm-smoke
Test: Existing tests pass.
Bug: 64146578
Change-Id: I62c55c99df86dba8c9ba9d9986049de1b1083234