We want a separate class for communicating HostAuth objects
to and from IEmailService. The issue is that the HostAuth object
was being used for both to and from the database, and to and from
IEmailService. This is dangerous because The Email app may change
out of band with the Exchange app, and may need to change the
format of HostAuth. This bit us before when adding OAuth.
Now communication to IEmailService is done using HostAuthCompat.
Change-Id: I2fb8c2bd8158f58a7bb9bc3dc83a7936948c718c
1d8fd9c054