Migrate Android sample to AuthTabIntent

[markmur]

What changes are you making?

https://developer.chrome.com/docs/android/custom-tabs/guide-auth-tab

Migrates the Android demo's Customer Account API authentication from an embedded WebView to the system browser:

• Uses AndroidX Auth Tab when the selected browser supports it, with a Custom Tabs/deep-link fallback.
• Uses the browser's shared session by default so sign-in and interactive logout participate in browser SSO.
• Keeps Customer Account integration explicit by continuing to pass the access token into Storefront cart buyer identity.
• Clears the cart and invalidates preloaded Checkout Kit state when authentication changes.

The OAuth flow is also hardened to make the sample a safer recommendation for consumers:

• Authorization Code + PKCE S256 with cryptographically random verifier, state, and nonce values.
• Strict redirect matching across scheme, host, port, and path, including duplicate parameter rejection and OAuth error handling.
• OIDC issuer, audience, authorized-party, expiry, issued-at, subject, and nonce validation for ID tokens returned by Shopify's token endpoint.
• Interactive browser logout using id_token_hint and post_logout_redirect_uri, while always clearing local credentials on completion, cancellation, or failure.
• Android Keystore-backed AES-GCM credential encryption and backup/device-transfer exclusions.
• Removal of token response and customer PII logging.
• Documentation recommending a verified HTTPS App Link for production redirect handling.

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Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

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Releasing a new Swift version?

• I have bumped the version in ShopifyCheckoutKit.podspec
• I have bumped the version in platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
• I have updated the SwiftPM/CocoaPods version snippets in platforms/swift/README.md (major version only)

Releasing a new Embedded Checkout Protocol version?

• I have bumped embeddedCheckoutProtocolAndroid in platforms/android/gradle/libs.versions.toml
• I have updated protocol/languages/kotlin/embedded-checkout-protocol/api/embedded-checkout-protocol.api if the public API changed

Releasing a new Android version?

• I have bumped checkoutKitAndroid in platforms/android/gradle/libs.versions.toml
• I have updated the Gradle/Maven version snippets in platforms/android/README.md

Tip

See the Contributing documentation for the full release process per platform.

[markmur]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Abstract customer accounts api to ShopifyCustomerAccounts #512
• Migrate Android sample to AuthTabIntent #513 👈 (View in Graphite)
• Migrate Swift sample from WKWebView to ASWebAuthenticationSession #511
• main

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