[local_auth] Allow empty localizedReason for Android

[Luciferx86]

localizedReason should be optional for Android. There could be use case where we only want to show a single piece of text on the biometric prompt. Currently showing 2 pieces of texts is mandatory.

Proposal

Only set the description parameter to the BiometricPrompt if a non empty String is passed in the call.

code:

AuthenticationHelper.java

use

.setDescription(strings.getReason().isEmpty() ? null : strings.getReason())

instead of

.setDescription(strings.getReason())

** screenshots attached in the original issue

Related: flutter/flutter#116885

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[stuartmorgan]

Thanks for the contribution!

This PR makes the Android implementation inconsistent with the public documentation of authenticate, which says (at both the app-facing and platform interface level) that the parameter must not be empty. That would mean that only people who read the implementation source would know that they could do the opposite of what the docs say, which is not how we would want to land this.

A complete fix needs to update the docs on those methods, as well as provide clients a runtime way of knowing whether or not the current platform requires a non-empty reason, via a support query. So this needs a new platform interface method, called something like requiresLocalizedReason(), and for that to be wired up in the app-facing package as part of changing the documention for authenticate.

[Luciferx86]

I understand that there needs to be a method like requiresLocalizedReason() for all the platforms, and for android in particular, this method will return false.

So to make this work, I would be required to update the documentation for authenticate method, as well as override this method in all platform implementations?

Also implementation for authenticate then becomes something like?

if(requiresLocalizedReason()){
assert(localizedReason.isNotEmpty);
}

Is this implementation correct, or you had something else in mind? @stuartmorgan