fix: ANRs in main thread

[martinpelli]

This PR attempts to fix ANRs issues mentioned in #629

Description

Fixes ANR triggered during app startup on Android when Intercom.initialize is called from Application.onCreate.

Intercom.initialize internally calls UserIdentityStore.loadUserFields using runBlocking over DataStore/Keystore operations. When invoked on the main thread, this blocks it for >5s on low-end devices (Tecno, Infinix), causing an ANR.

Changes

• initSdk now spawns a daemon background thread "intercom-init" to run Intercom.initialize off the main thread
• Added a CountDownLatch(1) that gates all method calls — any call arriving before init completes waits on a background thread, then posts back to main once the latch releases
• Fast-path added: once initialized, latch.count == 0L short-circuits the wait with zero overhead
• onMethodCall now delegates to a private dispatch(call, result) wrapped in runAfterInit — no per-method changes needed
• Added Intercom.initialized (Future<void>) on the Dart side that completes when initialize() finishes, for clients that want to explicitly await readiness

[deepak786]

@martinpelli Thank you for the PR. I will check.

Since you have put the initialization on a separate thread, I'm worried that it could trigger another issue

Intercom was not initialized correctly, Intercom.initialize() needs to be called in onCreate() in your Application class.

[martinpelli]

@deepak786 thanks! The thread is spawned from within Application.onCreate() and receives the Application instance directly. The Intercom SDK check validates that a valid Application context is passed, not that initialize() is called on the main thread. I have tested it and I don't see any problem, but I think until I test in production we don't really know.