Add echo cancellation to loopback audio capture

When sharing system audio in stereo, playback leaks back through the
capture path and remote participants hear an echo.  This adds a
WebRTC AudioProcessing (AEC) stage to the OpenAL capture loop inside
MixingAudioDeviceModule:

  - Create an AudioProcessing instance with echo_canceller enabled
    (mobile_mode) when the capture device opens in stereo.
  - Feed far-end (rendered) audio into ProcessReverseAudioFrame via
    LoopbackCaptureTakeFarEndLinux, which returns the estimated delay.
  - Run the captured microphone + loopback mix through
    ProcessAudioFrame before pushing samples to the collector.
  - Track capture activity with SetLoopbackCaptureActiveLinux so the
    far-end buffer is only maintained while capture is running.

Falls back to the existing non-AEC path when stereo capture is not
available or no far-end reference frame is ready.

Author: paterkleomenis