Fix iOS playback volume drop after re-muting

[ipavlidakis]

Summary

• Keep the iOS audio input graph alive when all send streams are muted.
• Prevent muting from stopping ADM recording, which recreated the AVAudioEngine as output-only and reduced playback loudness.
• Add an AudioState regression test covering mute changes without recording teardown.

Test plan

• git diff --check
• bundle exec fastlane ios build verbose:true zip_product_skip:true root:/Users/ipavlidakis/workspace/webrtc/src build_root:/Users/ipavlidakis/workspace/webrtc/src configure_google_client_skip:true skip_licenses:true maccatalyst_support:false sign_product_skip:true verify_signatures_skip:true

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch iliaspavlidakis/m145/fix-audio-output-gain-issue

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