The prefetched source was unnecessarily holding a lock while receiving data...

The prefetched source was unnecessarily holding a lock while receiving data from the network, sometimes blocking for seconds over 2G networks with corresponding drops in audio playback. Also, if the cache was full we'd report a cache duration of 0 instead of maximum size.

Change-Id: I46c8083f12f2dd728302095369197ca7e6b78bf8
related-to-bugs: 2492187, 2444425