[local_auth] Federate READMEs

[stuartmorgan-g]

Moves the platform-specific setup details from the main README to the implementation package READMEs, per our current best practice.

Minor cleanup as part of moving things:

• Simplifies iOS description and adds a link to the Apple docs.
• Removes the Java variant of the app setup instructions; new apps are overwhelmingly Kotlin, and Java users can extrapolate from the Kotlin.
• Removes an obsolete section about detecting face ID support on Android; we no longer support specific auth method detection on Android at all.

Part of flutter/flutter#182772

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩↩²↩³

[gemini-code-assist]

Code Review

This pull request moves platform-specific setup instructions from the main local_auth README to the implementation package READMEs for Android and Darwin platforms. As part of this move, the instructions have been updated: the Java example for Android was removed, an obsolete Android compatibility section was removed, and the iOS instructions were simplified. Package versions for local_auth, local_auth_android, and local_auth_darwin have been incremented.

My review identified a typo in an XML code snippet within the local_auth_android README, and a suggestion has been provided to correct it.