Extend stack management to other displays.

- Abandon ActivityContainer.startActivity() in favor of
IActivityManager.startActivityAsUserInContainer().
- Modify Am to test starting an activity on a container.
- Create a DisplayContext as the base context if the activity token
is on a different display.
- Test for home display in more cases when manipulating home stack.
- Rename mDisplayInfos => mActivityDisplays.
- Create new method for moving task to front of stack regardless of
which display it is on.

Change-Id: I4fcb83ae844c5839ee3e2722229623d1a80ed921