Using an executor with increased queue size for loading previews

An async task can be in the queue even if it has been cancelled. It
is removed only after it gets executed once. So the executor can get
blocked if there are some tasks which are running, and we keep posting
and cancelling preview load tasks.

Bug: 24306650
Change-Id: I836c8d53258542cc6f31952dff84323cc057437c