audio: support custom acdb for external device

add the specific external device setting in audio_platform_info.xml

For example:
<external_specific_dev>
    <ext_device name="SND_DEVICE_OUT_USB_HEADSET" usbid="18d1:5034" acdb_id="10" />
</external_specific_dev>

Bug: 117336751
Test: manual test ok
Change-Id: Ie9abcadd9bcd6d06c5d8f0a9807c3935ee4a1d35
Signed-off-by: Carter Hsu <carterhsu@google.com>
Signed-off-by: Robert Lee <lerobert@google.com>