fix: revert keychain dependency back to fork

[antsgar]

Reverting react-native-keychain dependency to use our forked repo.

• The fork has been updated with tweaks to package.json to make it work with yarn, so that the ios directory will be included when installing, and the lib directory will get built as well (see commit standardnotes/react-native-keychain@618c3e1 in the forked repo).
• This updated package.json fixes the issue with iOS biometrics, which was caused by the missing ios folder on install.