HAL stream format for mixer output threads must be stereo 16-bit PCM

Direct and tunnel output threads can support various HAL stream formats,
included encoded.  But currently there are stereo 16-bit PCM assumptions
in several places for mixer and duplicating output threads:
 - mMixBuffer and mixBuffer()
 - AudioMixer including resampler
 - FastMixer's mixBuffer
 - effects
 - NBAIO_Format
 - anywhere FCC_2 is used
 - possibly other places

Until those assumptions are removed, this CL enforces stereo 16-bit
PCM in mixer and duplicating threads at the place where the HAL format
is read.  It was already being checked in checkForNewParameters_l(),
but not in readOutputParameters().

Change-Id: Ibe344cc922743da234299097aa1bb1f54795cc9b