Restore channel volume of applications upon exit (#16312)
Closes #16292
Summary of the issue:
When sound split is enabled, if an application is closed while NVDA is still running, its channel volume would be preserved even after the app is restarted later without NVDA.
Description of user facing changes
N/A
Description of development approach
Catching on_state_changed notification and when new_state == "Expired" - this means that the application just exited - we restore both left and right channel to full volume.
