This patch moves the v4l2* source files to a src/video/v4l2 directory,
in order to facilitate the integration of other video-specific
libraries.

It also uses the Pimpl idiom to split the V4L2 specific-implementation
from video_preferences.cpp into v4l2/video_preferences_v4l2.cpp, and
rename the video_v4l2_list files and class to video_device_monitor_impl.

This patch also removes the v4l2 test since this class is now hidden in
the v4l2/video_preferences implementation. A test should be implemented
with the upcoming video device monitor class.

Refs #46837
Change-Id: I9920e20b2a873a772570c6bdd113aa45d37ce0bf