policy_hal: Function prototype correction for custom policy

- Ringtone is not heard when BT is connected because mute count
  for stream type ring is always greater then zero. The reason
  is when BT is connected duplicate output is created. Base policy
  does not handle mute count for duplicate output in stopSource
  properly. Custom policy stopSource handles this scenario but it
  is not called as signature does not match with base class
  stopSource

- Fix is to correct function signature for custom policy to make
  sure stopSource is called.

Change-Id: I8c0a99dbaae860a261ce2f15b6056d3d9c545568