extractor: Fast-track the stream analysis if we already have a skip condition

The sniffer already has an abort clause if the container is identified
as MPEG4 and the media source is caching (HTTP, typically). If we figure
this out early, there's no need to drill down into the individual streams,
and we can skip the deeper stream analysis, saving somewhere between 5 and
15 seconds in the identification process.

Change-Id: I64d33dc22262dd389e48b5f07047a8a1908f4ff3