[backport#15288] Remove use of uiInterface.LoadWallet in wallet code

Summary:
This also changes the uiInterface.LoadWallet signal argument type from
shared_ptr<CWallet> to unique_ptr<interfaces::Wallet> because CWallet is an
internal wallet class that shouldn't be used in non-wallet code (and also can't
be passed across process boundaries).

This commit does not change behavior.

1106a6fde4bfde31a16de45e4cc84ed5da05c5a4 (Russell Yanofksi)

Depends on D5867

This is a partial backport of Core PR15288

Test Plan:

cmake .. -GNinja -DENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Debug -DBUILD_BITCOIN_WALLET=OFF
ninja check-all
cmake .. -GNinja -DENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Debug
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5868