Load UserDictionary and AutoDictionary in a background thread.

This is to avoid ANRs during bootup, as some of the providers
may not have been initialized yet.

Refactored the ContactsDictionary and moved the async loading
code to ExpandableDictionary to share with the other dicts.

Bug: 2501133
Change-Id: I20393edb6fdf5df2f54ebac8dd04419a592177a2