audiodecoder: copy rate and channels from input before fixating output caps

audiodecoder: copy rate and channels from input before fixating output caps

For default caps generation when handling gap events that are sent
before any buffer, try to use caps that are closer to what upstream
provided to avoid fixating rate or channels to 1 as default.

So there are the steps:
1) Try to set rate, channels and channel-mask from upstream if provided
2) Fixate the rate and channels to the default rate and channels from
   audio lib
3) Fixate the caps just to be sure everything is fixed
4) If no channel-mask was provided and channels > 2, use a default
   channel-mask (taken from audioconvert code)

https://bugzilla.gnome.org/show_bug.cgi?id=722144