host/mobile: integrate GAPK based audio I/O back-end

This change introduces a new feature of the mobile application -
audio I/O support, which allows one to speak right from PC running
mobile through its ordinary mic and speakers.

The audio I/O is based on GAPK library, which relays on ALSA sound
framework. The API of GAPK implies to use the processing queues
(chains), which basically consist of a source block, several
processing blocks, and a sink block. So, there are two voice
processing chains:

  - 'pq_audio_source' (voice capture -> frame encoding),
  - 'pq_audio_sink' (frame decoding -> voice playback).

Both of them exchange frames from two dedicated buffers:

  - 'tch_fb_ul' - a buffer for to be played DL TCH frames,
  - 'tch_fb_dl' - a buffer for encoded UL TCH frames.

In its turn, both buffers are served by a new gapk_io_dequeue()
function, which is being called within the mobile_work() loop.

Change-Id: Ib86b0746606c191573cc773f01172afbb52f33a9

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