update Settings activity to use passed in blog id

Rather than always updating the settings for the currentBlog, now use
the ID passed in the intent extras.  Also add new utility method to get
the Blog object for a given ID.

git-svn-id: https://android.svn.wordpress.org/trunk@931 a6565c4b-a94d-4d28-bb26-aa9e5ab542c0