libstagefright: Use channel count if channel mask is zero.

-If the channel mask which is part of the meta data is zero use
 a valid channel count to update the channel mask.
-PCM offload playback for wav clips fails for some clips because
 channel mask part of the wave header is zero.

CRs-Fixed: 664374
Change-Id: I1bd5f2cc56fbc1eb9a6e04d219185690d3ce409c