aoc/alsa: audio capture from eraser

Added support to get audio capture stream from eraser for speech
recognition.

Usage examples:

- Choose capture mic source as ERASER
       tinymix "Audio Capture Mic Source" "ERASER"
       tinymix "EP1 TX Mixer ERASER_TX" 1

- Enable Eraser
       tinymix "Audio Capture Eraser Enable" 1

Bug: 172266161
Bug: 196896800

Signed-off-by: Xinhui Zhou <zxinhui@google.com>
Change-Id: I04333a695583c4cfa351cb121bd16bc52f5783b9