Clean up channel mask handling

Use type audio_channel_mask_t instead of uint32_t.

Use function audio_channel_out_mask_from_count() to convert from channel
count to channel mask instead of hard-coded assumption.

Rename 'channels' to 'channelMask' to avoid confusion with channel count.

Change-Id: I21a85e668458b773d108f95c2381eef9c4816251