client: new command 'update-prefs' for updating user preferences

Currently, only support updating the email address of user. The
who_am_i xmlrpc API is also updated to return email address in
additional.

Bug: 832397
Change-Id: If241eabd9a0284e6a3747d9a13306bbfdf5b9bf5
Merge "client: new command 'update-prefs' for updating user preferences" into develop