Rewrite AutocryptHeaderParser and AutocryptGossipHeaderParser in Kotlin

[shamim-emon]

Currently, AutocryptHeaderParser and AutocryptGossipHeaderParser are implemented in Java. For better Kotlin interoperability and idiomatic code, both classes should be converted to Kotlin with the following improvements:

1. Convert both classes from Java to Kotlin.
2. Maintain singleton pattern using private constructor + companion object.
3. Use idiomatic Kotlin features:

• when expressions for validation
• mapNotNull instead of manual loops
• Null safety (?., ?:, etc.)
• Avoid redundant operations (e.g., Base64 decoding)

4. Logging should be consistent using a dedicated TAG constant.
5. parseAllAutocryptHeaders/parseAllAutocryptGossipHeaders should accept Collection<String> for flexibility.
6. Preserve existing functionality and validation rules.
7. Add @VisibleForTesting annotations to parsing functions where applicable.