Add different ways of reading the dictionary file.

This change adds basic support for an external dictionary provider.
It adds methods for reading the dictionary itself from an asset in
the dictionary provider package directly, obtaining the file name
through the ContentProvider interface; it also adds a way of getting
the data through an InputStream and copying the file locally.

Incidentally this change also adds the code needed to listen for
updating the dictionary provider package and reloading it in time.

This change also goes hand-in-hand with Iab31db6e, which implements
the small closed part of this.

Issue: 3414944

Change-Id: I5e4fff99a59bb99dbdb002102db6c90e6cb41c8a