decodebin2: do not handle the next-groups list as if it was a single item

decodebin2: do not handle the next-groups list as if it was a single item

Decodebin2's chains store a next_groups list that was being handled as
it could only have a single element. This is true for most of the
chaining streams scenarios where streams change not very often.

In more stressfull changing scenarios, like adaptive streams, those
changes can happen very often, and in short time intervals. This could
confuse decodebin2 as this list was always being used as a single
element list.

This patches makes it handle as a real list, using iteration instead
of picking the first element as the correct one always.