audio: add 'useForVolume' field to 'gain'.

BUG: 144081761
Test: xmllint --noout --xinclude --schema \
      hardware/interfaces/audio/5.0/config/audio_policy_configuration.xsd \
      frameworks/av/services/audiopolicy/config/audio_policy_configuration*.xml
      xmllint --noout --schema \
      hardware/interfaces/audio/5.0/config/audio_policy_configuration.xsd \
      --xinclude out/target/product/*/vendor/etc/audio_policy_configuration.xml

Change-Id: I0ee9114f750186b479a2ebee5dfcd946d9ef3196