Don't update task to return to type for last task over home

Resizing docked stack to fullscreen size results in dismissal of
docked stack and moving of tasks to fullscreen stack. When tasks,
which were originally launched from home or recents, are inserted
on top of fullscreen stack, their mTaskToReturnTo value was
overwritten.
This CL doesn't allow to overwrite this field of task if there is
no next task in stack and it was launched from home or recents.

Also this reveals issue b/29342752. When app is moved to PiP mode,
system UI shows another overlay activity in separate task in PiP
stack with hint to user. In ActivityStarter#startActivityUnchecked
some logic assumes that new activity is launched from focused stack.
But PiP stack is not focusable, so we end up thinking that overlay
activity was launched from home stack. This sets mTaskToReturnTo of
overlay task to HOME_ACTIVITY_TYPE instead of
APPLICATION_ACTIVITY_TYPE.
This CL checks if stack id is specified and checks preferred stack
if available instead of currently focused one.

Bug: 29237751
Change-Id: I3a55b84d6a57be758705db51dec60592683e4e02