v4l2_codec2 encoder: Store C2 linear buffer inside bitstream.

Currently an FD is stored inside the BitstreamBuffer object. This CL
moves the C2 linear block buffer directly inside the BitstreamBuffer.
This makes management of output buffers in the encoder simpler as we
don't need to manually keep output buffers alive. This is also required
by subsequent CLs that introduce functionality to prepend H.264 SPS and
PPS NAL units to IDR frames in the video stream.

Since both the decoder and encoder make use of the BitstreamBuffer
class and both have different requirements some changes needed to be
made. The decoder uses a 'C2ConstLinearBlock' as bitstream buffers are
used as input and do not need to be modified. The encoder uses a
'std::shared_ptr<C2LinearBlock>' as the encoded output stream will be
written to a bitstream buffer object. To accommodate these requirements
the BitstreamBuffer object is split up into a 'BitstreamBuffer' and
'ConstBitstreamBuffer' object.

BUG: 161495502
Test: arc.VideoEncodeAccel.h264_192p_i420_vm
Change-Id: Icfd0a84ef09cf283510763c51f92fe83bc7166c2