The audio track was accidentally not participating in the prefetch since it...

The audio track was accidentally not participating in the prefetch since it wasn't started at the time prepare() was called. Also, properly report the cached duration even near the end when the source has no more data to fetch.

Change-Id: I66a92fec24c9bfb25f1c186f1c877127bae2b4f9
related-to-bug: 2444425