Rename PageFetchTask to SectionsFetchTask

Also fix tests. Also change the PageViewFragment to an
explicit state based system rather than an implict one based on
which variables are null.

Change-Id: I05d85190c24231d54b8525239d01be7370a73287
Make PageImages fetching adjust to screen density

Change-Id: Id8c9078c073f11882674c28a135a4123a2449ede