fix bug where the instance saving of the ChooseTypeAndAccountActivity

would only work in the case where onSaveInstanceState was called.

 - combined mExistingAccounts and mSavedAccounts into one field
 - set this field when starting the addAccount request
 - write this to the instance state if the request state
   is ADDING_ACCOUNT
 - read this field from the instance state, if any,
   when the request state is ADDING_ACCOUNT

Bug: 5459669
Change-Id: I5a7b4943de8e706cc8a21ff9f54fce4258f18b19