Fix weirdness when home task forces rotation

- Make sure to retain the state when divider goes through a configuration
change in order to avoid that nothing happens when entering multi-window.
Save the state in DividerState and use a handler that's independant of the
attached state.
- Don't allow home task to dictate orientation unless the docked stack is
minimized. This caused a lot of weird bugs because when docking a task,
home stack gets moved to front, and if home task is front of stack, it
temporarily might dictate the rotation but later not anymore so this
causes two rapid configuration changes which may cause a lot of weirdness.

Change-Id: I6a2308af893cd8413ee8801e5b964f6ddc0abd51
Fixes: 28943853