hal: add a single mixer control MIXER_SAMPLE_RATE_DEFAULT for a2dp

Audio not heard on a2dp playback in older devices
where different sampling rates for rx and tx is not supported

Add a single mixer control to update the sample rate irrespective of tx or rx,
update this when the tx and rx specific controls are not available

Change-Id: I9d365c9ab464a26ba6bef52ab2a1f6b4e10fe3dd