Convert all 5 mutable `var` callback fields on CallManager
(onAnswerReceived, onIceCandidateReceived, onNewPeerInGroupCall,
onMidCallOfferReceived, onPeerLeft) into a single
`sessionEvents: SharedFlow<CallSessionEvent>`.
Benefits:
- Eliminates the stale-callback window between Activity destroy and
recreate. No more nulling callbacks in onDestroy — the `closed`
flag on CallSession is sufficient.
- No mutable state shared between Activity and background code.
- CallActivity no longer wires or tears down callbacks — CallSession
subscribes to the flow in its init block.
- Type-safe sealed interface (CallSessionEvent) replaces 5 separate
lambda types.
The renegotiationEvents SharedFlow remains separate because
renegotiation has its own glare-resolution logic in CallSession.
Updated all 11 test sites in CallManagerTest to collect from
sessionEvents instead of setting callback vars.
https://claude.ai/code/session_019yNnDjGKmJb19gadmojq54
Claude
ac6d8eb417