SpdifStreamOut: override getAudioProperties()

This CL restores the behaviour of
PlaybackThread::readOutputParameters_l() that was introduced in
ca5e6143 for the SpdifStreamOut path,
but that was changed in 560637e3, specifically that format, sample rate and channel mask are assigned from the shim rather than the HAL directly.

Change-Id: Iaca53f823b862ead135dd4820e232098aed605c7