Stagefright: use get(Input/Output)Buffer in async mode

Non-async mode uses get(Input/Output)Buffers to get the
buffer handles at start; buffer-IDs obtained after
dequeue() are used to directly reference the handles.
In async mode, MediaCodec calls-back when an input/output
is available. Relying on the initial buffer-list can lead
to race conditions when buffers are allocated or changed.
Hence use get(Input/Output)Buffer to query the buffer
handle each time and just-in-time.

CRs-Fixed: 765324
Change-Id: Ifc0b7558564a57bd9316d0f80ae9cf88dfd6a1f3