Add WebView snippets

[cy245]

Added snippets for https://developer.android.com/identity/sign-in/credential-manager-webview. Some slight modifications:

• Combined first and second snippets since the former is a subset of the latter and it's awkward to break up a class declaration into 2 parts
• Updated "Initialize the WebView" section to include function header to make it clearer
• Combined last 2 snippets since they're part of the same function
• "INJECTED_VAL" is a very long string that is from the original app. While not part of a snippet, I've kept as-is for future reference

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• identity/credentialmanager/src/main/java/com/example/identity/credentialmanager/PasskeyWebListener.kt:25, tag android_identity_create_listener_passkeys
• identity/credentialmanager/src/main/java/com/example/identity/credentialmanager/PasskeyWebListener.kt:218, tag android_identity_javascript_reply_channel
• identity/credentialmanager/src/main/java/com/example/identity/credentialmanager/WebViewMainActivity.kt:19, tag android_identity_initialize_the_webview

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[neelanshsahai]

LGTM
Nit: Spotless defaults to 4 spaces per tab indentation. Some files in the project have 2 space indent, and some have 4 space indent.
After adding spotless, we can run a quick build post merging this

[cy245]

LGTM Nit: Spotless defaults to 4 spaces per tab indentation. Some files in the project have 2 space indent, and some have 4 space indent. After adding spotless, we can run a quick build post merging this

Thanks Neelansh. Let's run a quick followup CL to fix the formatting w/Spotless since it affects other files too.