audio: Make 'surroundSound' element optional

This element makes sense only for TV platforms. The deserializer
treats it as optional. Update the schema to match this.

Bug: 116356348
Test: xmllint --noout --xinclude --schema \
      hardware/interfaces/audio/5.0/config/audio_policy_configuration.xsd \
      frameworks/av/services/audiopolicy/config/audio_policy_configuration.xml

Change-Id: I822c3ff10a6becc859d3d44fd58ef2ebcd3d40b1