Post all CM Home Api Card updates to a single background thread.

Creating new AsyncTask instances for various updates from API Content
Observers created a variety of race conditions. Processing of updates
must occur in order. Use a single background thread and post all card
operations as a runnable to a Handler running on this thread.

Change-Id: I6f79e9e2c846cc5a59a3e5e28901b2e7795fa639