Extract monitoring orchestration from MonitorService

[d4rken]

Summary

• Extract VolumeEventDispatcher — moves volume event priority dispatch out of MonitorService into a standalone, testable class. Replaces Android SparseArray with groupBy/toSortedMap and fixes CancellationException swallowing.
• Extract MonitorOrchestrator — moves flow wiring and shutdown heuristics out of MonitorService. Uses an internal child Job for lifecycle control instead of cancelling the caller's scope. Captures the initial device snapshot once to avoid a race condition.
• Slim MonitorService down to a lifecycle shell (173 lines, 6 injections) — foreground promotion, session tracking, teardown, and notification updates via callback.

Test plan

• VolumeEventDispatcherTest — 5 tests: priority ordering, parallel execution, error isolation, CancellationException rethrow, empty set
• MonitorOrchestratorTest — 9 tests: bootstrap sequence, 3 shutdown heuristic branches, device list updates, 3 failure isolation lanes
• assembleFossDebug compiles
• Manual: connect/disconnect Bluetooth device, verify volume adjustment and service shutdown unchanged