Fix multi-ledger switching and assign existing posted DID as public

Follow up on #790
- Fix and enable ledger switching [currently, it errors out]
- Right now, when switching the ledger it creates a new sub-wallet DID and assigns that as public DID. This is not ideal, if there is an existing DID posted on that ledger using tenant-ui then it assigns that existing DID as public DID instead of creating a new one.

Start with the context in issue #790 and reproduce the current multi-ledger switching error. Trace the ledger-switching flow and DID assignment behavior; done means switching succeeds and an existing DID posted on the selected ledger through tenant-ui is used as the public DID instead of creating a new sub-wallet DID.

Written by the indexing model from the issue text.