[backport#16944][wallet] ListCoins: include watch-only for wallets without private keys
[backport#16944][wallet] ListCoins: include watch-only for wallets without private keys

Summary:
This makes them available in GUI coin selection.

https://github.com/bitcoin/bitcoin/pull/16944/commits/40537f090907f81ba885edb7dff1558382976912

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Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16944 | PR16944]]

Depends on D7292

Test Plan:
  ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7293

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