decodebin: Simplify adaptive streaming demuxer code a bit
decodebin: Keep the number of buffers after an adaptive streaming demuxer lower

Usually these buffers are multiple seconds large, and having a maximum
of 5 buffers in the multiqueue there can use a lot of memory. Lower
this to 2 for adaptive streaming demuxers.
decodebin: Buffer up to 5 seconds in multiqueue buffering mode

2 seconds might be too small for some container formats, e.g.
MPEGTS with some video codec and AAC/ADTS audio with 700ms
long buffers. The video branch of multiqueue can run full while
the audio branch is completely empty, especially because there
are usually more queues downstream on the audio branch.