Use native incoming call notification style with Answer/Decline actions

[pfurovYnP]

Motivation

• Incoming calls were delivered as ordinary notifications without native call UI affordances, causing a worse UX compared to other messengers. This change aims to present incoming calls as proper incoming-call notifications with quick answer/decline actions and full-screen intent behavior.

Description

• Build incoming call notifications using NotificationCompat.CallStyle.forIncomingCall(...) and a Person caller to enable native incoming-call presentation and richer actions in NotificationWorker.kt.
• Add explicit Answer and Decline PendingIntents where Answer launches CallNotificationActivity with KEY_CALL_VOICE_ONLY and Decline sends a broadcast to dismiss the ringing notification.
• Add IncomingCallDeclineReceiver to handle decline actions by cancelling the ringing notification and register it in AndroidManifest.xml.
• Update CallNotificationActivity to detect when it was opened via the notification Answer action (via KEY_CALL_VOICE_ONLY) and automatically proceed to CallActivity.

Testing

• Attempted to compile Kotlin with ./gradlew :app:compileDebugKotlin, which failed because the task name is ambiguous in this multi-flavor project (requires a specific flavor task). (failed)
• Attempted flavored compile with ./gradlew :app:compileGplayDebugKotlin, which failed in the environment due to missing Android SDK (ANDROID_HOME / sdk.dir). (failed)
• Static inspection and local execution of search/replace/format steps were performed to validate code insertion and manifest registration; no unit/instrumentation tests were executed due to unavailable SDK in the environment. (no run)

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Codex Task