adaptivedemux2: Improve minimum buffering threshold

adaptivedemux2: Improve minimum buffering threshold

Previously the minimum buffering threshold was hardcoded to a specific
value (10s). This is suboptimal this an actual value will depend on the actual
stream being played.

This commit sets the low watermark threshold in time to 0, which is an automatic
mode. Subclasses can provide a stream `recommended_buffering_threshold` when
update_stream_info() is called.

Currently implemented for HLS, where we recommended 1.5 average segment
duration. This will result in buffering being at 100% when the 2nd segment has
been downloaded (minus a bit already being consumed downstream)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3240>