ActivityChooserModel does not handle package changes on the thread that created it.

1. The model is using a package monitor to observe changes in
   packages (adding/memoving/etc) to update its internal state
   for which it then notifies its clients. However, the monitor
   is called from a binder thread and the work has to be off
   handed on the thread that created the model. Hence, clients
   do not need to worry about using the the model from the UI
   thread since the change callback may touch the UI from
   another thread.

bug:6386152

Change-Id: I882a0f4104907f64de64a95ece585052bba457d6