More movement from ActivityStack to Supervisor

- Move showAskCompatModeDialogLocked from ActivityStack to
ActivityManagerService.
- Keep track of userId in ActivityStackSupervisor and initialize
new ActivityStacks with it.
- Move realStartActivityLocked to ActivityStackSupervisor except
for the part that actually updates the ActivityStack.

Change-Id: I7467302f1249fee2ea3cf027a49e0894c9a2213d