Only preload selected pages

Otherwise if only some pages are selected all (non-selected) pages in
between are preloaded.

Test: Selected only some pages of a large document and observed which
      pages got preloaded
      cts-tradefed run cts-dev -m Print
Fixes: 62296301
Change-Id: I3a97cfa6991e5a95ff73628c4bb540c629160c0c