A2dp: Send default channel mapping for TWS+ mono.

Audio loss observed when encoder configured with mono mode.

This occurs because when configuration of encoder with mono mode
happens 0 channel mapping is sent to ADSP which rejects it as invalid
channel mapping.

To avoid this send default stereo channel mapping for both mono and dual
mono case.

CRs-Fixed: 2528391
Change-Id: Ie8d04965fb43f1beea747b23ae86464f943afdae