Bug 1676942 - Refactor RunOnBackgroundThread r=nika

RunOnBackgroundThread has been renamed to RunOnBackgroundThreadAndResolve. The
body dealing with calling InvokeAsync for calling a function on the background
event target and returning a MozPromise has been refactored into a new
function, RunOnBackgroundThread.

This allows us to have more complicated IOUtils methods that do not simply
resolve their promises to whatever is returned by the background methods.

Differential Revision: https://phabricator.services.mozilla.com/D99002