Don't delete video node factories owned by the plugin loader.

Make the fallback factories members instead of creating on the
heap so there's no need to delete members of the m_videoNodeFactories
some of which may be shared with other current and future video
outputs.

Change-Id: I3d2e32e52479b12ff64e31a2ed527336d94a191b
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>