Add use_v4lplugin flag to enable v4l2 library
Dynamic load v4l-utils related libraries in unittest.
BUG=
405861
TEST=use video_encode_accelerator_unittest to make sure
v4l2_open and plugin_init is called. Related shared libraries are from
chromeos. When use_v4lplugin=0, unittest doesn't open v4l2 libraries.
Committed:
https://crrev.com/d909464cbdcf7d4f80aff4009d9796632657dfb8
Cr-Commit-Position: refs/heads/master@{#310701}