Defer MixpanelFlutterPlugin registration and initialization to prevent ANRs

[jaredmixpanel]

Defer MethodChannel setup from onAttachedToEngine into handleInitialize(...), so the channel is only created when Dart calls initialize(). This lazy registration prevents blocking the main thread during engine startup (including headless/Firebase isolates) and eliminates the ANR.

Key Changes
• Move new MethodChannel(...) + setMethodCallHandler(this) into handleInitialize() guarded by channel == null && flutterPluginBinding != null
• Leave onAttachedToEngine() free of any codec or messenger work
• Ensure flutterPluginBinding is stored on attach for later use in initialization

[Copilot]

Pull Request Overview

This PR defers the registration and initialization of the MixpanelFlutterPlugin to prevent potential ANRs during plugin registration. It implements lazy initialization for the MethodChannel, stores the FlutterPluginBinding for later use, and ensures proper cleanup in onDetachedFromEngine.

• Lazy initialization of the MethodChannel in handleInitialize
• Storing the FlutterPluginBinding for deferred channel registration
• Cleaning up channel and binding references in onDetachedFromEngine

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
android/src/main/java/com/mixpanel/mixpanel_flutter/MixpanelFlutterPlugin.java	Defers channel registration and adds lazy initialization to avoid ANRs
README.md	Updates Markdown formatting for consistency in the “I want to know more!” section