hal: set bt sco sample rate from hal instead of mixer path xml

Configure the bt sco device sample rate explicitly from hal and
don't reset it during disabling.
The initial value of mixer "BT SampleRate" will also be removed.
This is to avoid resetting sample rate to 8K for the scenario
of switching SCO to A2DP, causing A2DP playback failed.

Change-Id: I99e254b7b97f9b4abb21249b8e31b99eb401adfb