aoc/alsa: support aec ref source selection for eraser

This is to support selecting a differennt aec ref signal for eraser.
For example, on some device, the mic input signal is from
the builtin pdm mic, but the audio playback signal might be sent to
a usb device.

Usage example:
	Set the reference soruce as default:
	tinymix "Eraser AEC Reference Source" Default

		tinymix "Eraser AEC Reference Source"
		Eraser AEC Reference Source: >Default SPEAKER USB BT

        Set the reference soruce as USB playback:
		tinymix "Eraser AEC Reference Source" USB

		tinymix "Eraser AEC Reference Source"
		Eraser AEC Reference Source: Default SPEAKER >USB BT

Bug: 206659028

Signed-off-by: Xinhui Zhou <zxinhui@google.com>
Change-Id: Ia006c88eb5bdb2a90ba495bdb914fd1e5d8d73f1