frameworks/av: Fix channel mask if it is zero

-Currently, channel mask is set to stereo, in WAV extractor, when it's
value is zero. Audio player handles such case by setting appropriate
channel mask, based on number of channels, when channel mask is zero.
-By removing the redundant channel mask setting, correct
number of channels are set for non-stereo clips.

CRs-Fixed: 727240
Change-Id: I87b508b54a2016add7bf7274f6b2d4029db51ab3