Prevent occurences of clipping in loud downmixed scenes

The downmixer effect is a mixer, wich, like the platform mixer,
  hard-clips. To prevent clipping during loud scenes across multiple
  chanels, apply a fixed attenuation on the result of the downmix.
  Note that this attenuation is lower the theoretical required
  attenuation (except for quad and surround) but is adequate for
  most content.

Bug 4280902
Change-Id: Iec520e9f1367284d6102633f532429361717692d