Thumbnail Cache - check canceled status on the right thread.

The cache was checking the canceled status on the background
thread, but the cancel call was being made on the main thread.
This was leading to canceled requests still delivering this thumbnail
in some cases.

Bug: 159840851
Test: local build and non-repo of bug
Change-Id: I9a3556f6570eee1db39ebec202c115d58010d7f8