[mcp-account-manager] Use service name and account ID for telepathy account name

The traditional account names generated with the username don't fit us
well, especially since the username often isn't actually available when
creating an account. Also, they can be reused for different accounts
after being deleted.

We can use the service name and the account ID to generate a telepathy
account name instead, which is properly unique and reproducible. This
same technique is used by other account managers, such as GOA.

With this change, we get names like:
gabble/jabber/facebook_2dim_31
gabble/jabber/google_2dtalk_28

the '_2d' is telepathy's way of escaping a -, which is annoying but necessary. It could be simply replaced with an _, but that would slightly break the pattern usually used for these names.