Fix bug 5563362: failed to initialize external account type

The contacts.xml parser has had a bug that it processes each ContactsDataKind
tag twice,for START_TAG and for END_TAG.

When processing for END_TAG, because an end tag doesn't have attributes
it created an empty DataKind, with null mimetype.  (This bug existed even
before the EditSchema change, meaning MR0 still has this issue.)

Before MR1 this null mimetype was just not used anywhere, so it was okay.
But in MR1 we have the duplicate mimetype check, and it will complain
if a contacts.xml has more than one ContactsDataKind therefore has multiple
"null" mimetype definitions.

Made sure to only process for START_TAG.  Also cleaned up existing code
with a similar check.

Also included the package name in the error message for duplicated mimetypes.

Bug 5563362

Change-Id: Ib68fb0dca03784c4d7976ac8bf68b1dce5305b03