Fix bug #13058470 Cannot add account -- Settings app crashes

- revert back to an Activity instead of a fragment. This
is a straight reverse of the changes introduced for trying
to have a Fragment.

Basically ChooseAccountActivity was previously an Activity for
choosing an account type. If the list of account types was containing
only one item, then the ChooseAccountActivity was just a pass thru
and was falling back to the only account authority available.

In the current reported bug, this was happening by disabling the
Email app and thus having only the GoogleAuthenticator as an
Authority.

Then in the onCreate() the ChooseAccountFragment was seeing that
there was only one account authenticator and issuing a BACK button
press. Too bad, this was done into a non finished Fragment transaction
and leading to a crash.

All in all, we NEED to have an Activity and cannot use a Fragment
in that case.

Change-Id: I4a867a25fe9580929ec50a6775105adac1f88c52