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
diff --git a/audio/6.0/config/audio_policy_configuration.xsd b/audio/6.0/config/audio_policy_configuration.xsd
index 0dc89bb..d0f80ea 100644
--- a/audio/6.0/config/audio_policy_configuration.xsd
+++ b/audio/6.0/config/audio_policy_configuration.xsd
@@ -446,6 +446,7 @@
                     <xs:attribute name="stepValueMB" type="xs:int" use="optional"/>
                     <xs:attribute name="minRampMs" type="xs:int" use="optional"/>
                     <xs:attribute name="maxRampMs" type="xs:int" use="optional"/>
+                    <xs:attribute name="useForVolume" type="xs:boolean" use="optional"/>
                 </xs:complexType>
             </xs:element>
         </xs:sequence>