Add to SFOS local manifest for droidmedia with audiopolicy

<!-- droidmedia fork needed for audiopolicy support, see this PR ;
     https://github.com/sailfishos/droidmedia/pull/11 -->
  <project name="platform/external/libxml2"              path="external/libxml2"
           remote="aosp"                                 revision="nougat-release" />
  <remove-project name="sailfishos/droidmedia" />
  <project name="vknecht/droidmedia"                     path="external/droidmedia"
           remote="github"                               revision="master" />