save macos keys in keychain under app.bundle.id

[zeroflaw]

Issue: #43 (comment)

Removing keys in macOS is not obvious because they are not saved under the app.bundle.id. This pull request fixes that.

[hpoul]

thanks for the PR, but this is a pretty heavy breaking change.. i think it would be better to allow configuration of the service name? (maybe keeping the default to flutter_biometric_storage 🤔️)

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