audiopolicy: do not update channelMask for voice input source

It's not needed to update the channel mask based on source because
the source is sent to audio HAL through set_parameters() and if source
equal to VOICE_CALL does not mean that two channels need to capture.
If recorder app selects AMR as encoding format but source as RX+TX means
both RX and TX are captured in ONE channel. So use channels set by the app
and use source for the type of capture (RX only, TX only, or RX+TX).

Change-Id: Ic34c9c994713ac9b79da19d61e61ca91dc210ae6