obexd: Return dummy_data instead of int in phonebook-dummy

obexd: Return dummy_data instead of int in phonebook-dummy

There are two functions in phonebook-dummy that were returning
'int's instead of 'struct dummy_data'

phonebook_create_cache
phonebook_get_entry

As a result, when an obex-client sends the GetSize command, the obexd
on the server segfaults.

Fix this by storing the id and returning the dummy_data struct.

The GetSize test now passes correctly.