Limit the number of expensive thumbnails processed at the same time

Depends (soft) on Ibdf8ec222f9756d70de2bed7ff14913351dc394b
and Ic737673b5a31974d866b9bf5afa4b4036baa03bb.

Bug: 65691
Change-Id: Ic62984e0f4a761642b2bdd1bfa362301ed94c284
Merge "Limit the number of expensive thumbnails processed at the same time"