wifi: rename mode 'ON' to 'VOICE'.

To improve clarity and accommodate future functionalities,
we should rename the existing 'ON' mode to 'VOICE'.
This change establishes 'VOICE' as the fundamental mode for VOIP operations.
Additionally, it paves the way for adding future modes, such as 'VOICE_VIDEO',
as the system evolves.

Bug: 295885471
Test: TH

Change-Id: Ie3b54f02cf3c73b5a9c2a805ec36703dfd541666