feat: Add Android push notification support to Expo config plugin

[Rosie-Kennelly-1]

Why?

The Expo config plugin automates iOS push notification setup (since PR #191) but has no Android equivalent. This means Expo developers' end users silently never receive push notifications on Android when a support agent replies to their conversation — and the only workaround is manually creating native Kotlin files, which defeats the purpose of using Expo.

How?

A new config plugin generates a Kotlin FirebaseMessagingService at expo prebuild time and registers it in AndroidManifest.xml, following the exact pattern already proven in the with-notifications example. The service routes Intercom pushes to the SDK and passes non-Intercom messages through for other handlers (e.g. expo-notifications). The plugin also conditionally adds firebase-messaging to the app module's build.gradle when not already present — needed because the native module declares it as implementation (private to the library), but the generated service in the app module requires it on its compile classpath.

Also fixes a pre-existing bug in withPushNotifications.ts where config was passed instead of newConfig to appDelegate and infoPlist, breaking the plugin chaining.

Decisions

• Always applied (not opt-in) — matches how iOS push setup works today. The plugin runs unconditionally when @intercom/intercom-react-native is in your Expo plugins.
• android:exported="false" on the service — more secure than the "true" used in the with-notifications example. Firebase delivers messages internally; the service doesn't need to be externally accessible.
• android:priority="10" on the intent filter — explicitly ensures Intercom's service handles FCM events before lower-priority services like expo-notifications (which registers at -1).
• Conditional gradle dependency — firebase-messaging:24.1.2 is added to the app's build.gradle only if not already present. The native module declares it as implementation (private), so the app module needs its own copy for the generated Kotlin to compile.
• Idempotent — running expo prebuild multiple times won't duplicate the manifest service entry or the gradle dependency.
• Existing FCM service detection — if the manifest already has a FirebaseMessagingService registered (from manual setup or another SDK), the plugin skips registration and warns. The generated .kt file is still written since withDangerousMod (file generation) runs before withAndroidManifest (conflict detection) in Expo's mod pipeline, and cross-mod dependencies would add unnecessary complexity. The orphaned file is harmless — it's an unregistered source file in the ephemeral android/ directory that expo prebuild regenerates from scratch.

_{Generated with Claude Code}

[Rosie-Kennelly-1]

Superseded by a stacked PR series for easier review:

• fix: correct config chaining in withIntercomPushNotification PR #383 — bug fix: config chaining
• feat: generate Android FirebaseMessagingService for Expo push notifications #385 — Kotlin file generation + gradle dependency (Part 1)
• feat: register Android push service in manifest and wire into plugin #384 — manifest registration + plugin wiring (Part 2)