Added unlink identity to AccountManager

This adds functionality to remove identities, we remove the
identity from AccountExternalIds and we make sure the preferred
email is in sync with it. We also flush the caches.

We need this function for issue 1095, which adds setting account
details that includes deleting email(s).

Change-Id: Ia2bc785258f475a66befaa0e79893b26ca526c54
4 files changed