v4l2_codec2 encoder: Add support for configuring the bitrate mode.

This CL adds support for the C2_PARAMKEY_BITRATE_MODE parameter to the
v4l2 encoder. This parameter allows configuring the bitrate mode used
to encode a video.

The supported bitrate modes are:
- C2Config::BITRATE_VARIABLE
- C2Config::BITRATE_CONST

Note: Currently the C2 framework does not support configuring the peak
bitrate, which is used when the bitrate mode is set to VBR. Instead we
set the peak bitrate to a multiple of the target bitrate.

Note: Submit after support for V4L2_CID_MPEG_VIDEO_BITRATE_PEAK and
V4L2_CID_MPEG_VIDEO_BITRATE_MODE has been added to the virto encoder in
crrev.com/c/2944506.

BUG: 190336806
BUG: 181514834
Test: arc.VideoEncodeAccel.h264_192p_i420_vm
Change-Id: I95d8f9921c1ba475ea8c65760d3c18e5e2818d5e