Add sound split disabled mode (#16331)
Closes #16287
Summary of the issue:
Disabling sound split still could touch channel volumes.
Description of development approach
1. If sound split mode is disabled at startup, we're not touching any volumes.
2. If user toggles to disabled mode, then first we set mode to NVDA_BOTH_APPS_BOTH to effectively restore the volumes, then set sound pslit state to off.
