Fix race condition in initializing downmix

The code that initializes downmix GUID searches for an effect of
the right type.  If two or more AudioMixers are created, there is a
possibility that the GUID will temporarily have the wrong value.

Now the initialization is only done once, regardless of how many
AudioMixers there are.

Other minor changes:
 - use naming convention of leading 's' for static member variables
 - log message for no downmix effect is now a warning instead of an error,
   since it is not an error until there is actual content to downmix
   without a downmix effect

Change-Id: I33ab0e51cf9d24ecaf8ae868883736b0772178c4