Page content sometimes missing in preview when scrolling.

Removed the restriction for the page content providers to
be singletons as the RecyclerView may hold on views with
providers and ask us to bind other views for the same page.
Note that the views that RecyclerView is holding will be
soon rebound or recycled just they are not at the time we
bind other views for the same position. We were guarding
against this but caused some pages not appearing sometimes
on scrolling. Also now keeping the media size and magins
of recycled page view (will be updated if needed on
rebinding) or avoid changes of the background to null as
setting it to not null later causes an undesired layout pass.

bug:17537922

Change-Id: If029f6c413e15d4f5c1ba1ac5e4d52ad2cad5904