Allow calling runOnPlatformThread() from helper isolates (but not for background service isolates). At the moment runOnPlatformThread() is only supported on the UI isolate. This is mainly just a matter of plumbing. There's a bunch of state that the platform thread needs access to, to create the platform isolate, that's currently only available on the UI isolate (primarily the task runners).

This issue is a spin off from #136314