stagefright: Add a prefetching source

Add a prefetching source which pre-fetches buffers from
upstream source in a low priority context.
This decouples the caller of read() from the underlying source,
effectively making read() non-blocking as long as there are
already prefetched buffers available.

Change-Id: Ie5338c825dbdab9d88989059204594dc66bb2ac7
libstagefright: Fix wrong ifdef

Change-Id: Ic99d95bd1d19036712f422a8c1f673bda15e2964