message: Remove defvalue from keepContext
diff: If the value is MOBILE_APP, the system shall switch to the mobile media app that issues the setter RPC.
  </description>
  </param>
  - <param name="keepContext" type="Boolean" defvalue="false" mandatory="false">
  + <param name="keepContext" type="Boolean" mandatory="false">
  <description>
  This parameter shall not be present in any getter responses or notifications.
  This parameter is optional in a setter request. The default value is false if it is not included.
  