[local_auth]: Add option to only use strong biometric authentication systems on Android

[chrisparton1991]

Fixes flutter/flutter#81169.

Adds a new strongAuthenticatorsOnly flag that instructs Android to only allow secure biometric authentication mechanisms.

For instance, many Android devices use weak implementations of facial recognition, and in my experience most apps will only allow me to authenticate via fingerprint.

This change is backward compatible; omitting the flag will allow weak authentication as before.

Photo of strongAuthenticatorsOnly=false and strongAuthenticatorsOnly=true. This couldn't be a screenshot sorry, my phone won't allow me to screenshot the auth popup.

Note: I'm not sure how this will fit into the availableBiometrics method. At the moment I haven't made any changes there.
If the user has no secure methods available, a NoHardware or NotEnrolled should be returned.

List which issues are fixed by this PR. You must list at least one issue.
New feature.

If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].
N/A

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[stuartmorgan-g]

Thanks for the submission! We’re currently working through a large backlog of PRs, and this will require non-trivial review, so it will take some time before we’re able to review it. As explained in CONTRIBUTING.md, votes for the corresponding issue are the primary way we’re prioritizing non-trivial reviews, so we encourage you to file an issue and associate it with this PR.

Note: I'm not sure how this will fit into the availableBiometrics method. At the moment I haven't made any changes there.

See discussion in #3763

[chrisparton1991]

Hi @stuartmorgan, I've raised an associated issue, sorry I didn't see that earlier!

[stuartmorgan-g]

Given that we're planning on making breaking changes to the plugin API to have it operate in terms of weak and strong on Android rather than specific biometrics, as discussed in the issue linked above, I'm going to close this; a separate boolean won't be necessary in that approach, and we don't want to add something to the API that we're planning on removing shortly. Thanks again for the contribution though!

[chrisparton1991]

No problem, appreciate the update and the hard work!

[charafau]

@stuartmorgan Is there an issue for this somewhere? I'm also interested in this and would like to monitor the progress. Thanks!

[stuartmorgan-g]

See flutter/flutter#45497 (comment)