Watch CallKit events natively to auto-restore WebRTC audio session

[Copilot]

When a phone call interrupts a WebRTC session, the iOS audio session is taken over by CallKit/system and WebRTC cannot recover it automatically on release.

Changes

Native CallKit observer (PluginRTCAudioController.swift)

• import CallKit; class now inherits NSObject and conforms to CXCallObserverDelegate
• CXCallObserver registered in init() on DispatchQueue.main
• callObserver(_:callChanged:) tracks active call count:
  • First call connects → RTCAudioSession.sharedInstance().audioSessionDidActivate(AVAudioSession.sharedInstance())
  • Last call ends → audioSessionDidDeactivate + initAudioDevices() + restoreInputOutputAudioDevice()

Covers both regular PSTN calls and CallKit VoIP calls — CXCallObserver observes all device calls.

CallKit framework (plugin.xml)

<framework src="CallKit.framework" />

JS escape-hatch APIs (js/iosrtc.js)

For apps with their own CXProvider that need exact timing control over audio session transitions:

cordova.plugins.iosrtc.audioSessionDidActivate();   // call inside provider(_:didActivate:)
cordova.plugins.iosrtc.audioSessionDidDeactivate(); // call inside provider(_:didDeactivate:)

These forward directly to RTCAudioSession.sharedInstance().audioSessionDidActivate/Deactivate.

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