Fix local VAD flickering on cleanup

[Critteros]

Description

useLocalVAD polls getLocalTrackAudioLevel in a setTimeout loop. On cleanup it calls clearTimeout, but that can't cancel a poll that's already mid-await. The in-flight promise still resolves after
cleanup runs, calls setIsSpeaking, and schedules a fresh setTimeout so isSpeaking flickers and the poll loop leaks past the disabled state.

Fix: added an AbortController and check signal.aborted around each await boundary. The poll exits cleanly on cleanup and won't schedule another timeout.

Motivation and Context

isSpeaking was flickering when the hook got toggled off or the component unmounted, and a stale poll could keep running against aborted state. Users saw the speaking indicator flash briefly after going
silent or leaving the room.

Documentation impact

• Documentation update required
• Documentation updated in another PR
• No documentation update required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)