Change preference defaults to match to Android port

- layout.imagevisibility.enabled set to true so that we don't
  unnecessarely decode offscreen images
- memory.free_dirty_pages set to true now that jemalloc works
Merge pull request #16 from rainemak/updateDefaultPrefs

Change preference defaults to match to Android port
Merge branch 'embedlite' into embedlite_compositorReworkProtectedVersion