Downmix support for 5.1 and quad with side channels

In DOWNMIX_TYPE_FOLD mode, add support for variants of quad and
 5.1 channel masks where the "side" channels are used instead of
 the "back" channels. These variants have the same channel order,
 so they can share the same downmix function.

Change-Id: I5f9cb7cb120927189de680c2a968d6136dc1fc43