stagefright-plugins: Only do as much sniffing as necessary

stagefright-plugins: Only do as much sniffing as necessary

 * Stagefright will now pass on the current knowledge of the stream
   instead of zeros/nulls, let's take advantage of it.
 * Don't bother sniffing if the confidence is already high.
 * Check how much info avformat_open_input found before we continue.
   For formats with headers, this is all we need and our latency is
   reduced to 10s of milliseconds vs. potentially seconds.
 * Add timing metrics in the debug prints.

Change-Id: I476fc6723c070e03a77c6aaa0f8198064a15af2c