avcenc: Prepend startcodes to output buffers

This makes the encoder output data that is similar to what
actual HW encoders output. This form is also what is expected
by virtually every other component - the only exception
seems to be stagefright's MPEG4Writer which happens to work
with this format.

This also makes the encoder's output work in other decoders
when passing through directly. If feeding the encoded data
straight through to a decoder (such as in the EncodeDecodeTest
CTS test), the output from OMX.google.h264.encoder cannot be
decoded by e.g. OMX.Nvidia.h264.decode on a Nexus 7 or
OMX.SEC.avc.dec on a Galaxy S3.

Change-Id: I6e16433a4a66936e3080723c8233681f53bd55e5