Don't set 'download_video' field on importing video from xml until expli...

...citly set

TNL-324
@zubair-arbi I think it'll break old courses that don't have `download_video` field, but have `source`.

See requirements from https://github.com/edx/edx-platform/pull/2045 :
Alternate sources = AS(`html5_sources`), Download video = DV(`source`).
a) If DV exists and DV != AS: show DV field on front-end as not-editable and clearable. Dropdown is a new field `download_video` and it has value True. Student downloads DV. If teacher want to change DV, he should clear DV field and put video in AS.
b) If DV is cleared it is not shown anymore.
c) if DV exists and DV in AS, do not show DV field, dropdown has value True.

If the DV is the same as one of the AS videos, the DV link shouldn't be used. The AS should be used instead, so that we do not force authors to delete material in order to revise unless we absolutely must.