[e2e][sl4f] Add template audio facade

This adds a simple set of sl4f endpoints for controlling audio
on a DUT.  It currently supports Injecting audio into the input
path and Saving the output from output path.

Test: PutInputAudio endpoint
 {
   echo -n '{"id": "", "method": "audio_facade.PutInputAudio","params": "'
   cat test.wav | base64 -w 0
   echo -n '"}'
 } > payload.json
 curl -X GET [`buildtools/linux-x64/dev_finder list`] -d "@payload.json";

Test: GetOutputAudio endpoint
 curl -X GET [`buildtools/linux-x64/dev_finder list`] \
      -d '{"id": "", "method": "audio_facade.GetOutputAudio",
          "params": null}' \
 | python3 -c "import sys, json; print(json.load(sys.stdin)['result'])" \
 | base64 -d > audio_capture.wav

Change-Id: I0c52f1bd7723b346771308731722bcc57a668b04