Page content sometimes disappears when scrolling a long doc in print preview.

The operations of the remote renderer have to be performed in order, open,
render some pages, close. One of the tasks was executed on the wrong executor
resuling in a race and an occassional bad state.

Also fixed a NPE if the preview list is flinged and then the user presses
back.

bug:17537922

Change-Id: I5048078ba2b875a2a8335f3a4324afaa34d014a2