taskReturnToType be gone!

The activity hierarchy now maintains visual ordering since most task
have their own stack, so no need for taskReturnToType complexity.
Also, we handle the API FLAG_ACTIVITY_TASK_ON_HOME by moving the
home stack forward whenever a stack with the root task intent with
that flag is movedToFront.

Bug: 64146578
Fixes: 67931573
Test: go/wm-smoke
Test: Existing test pass.
Change-Id: I465df9cb55830ac27bba330b221f730c6f1e9450