video: convertframe: Shut down pipeline asynchronously via the thread pool

If we use the main loop it might happen that the caller (e.g. our unit
test) already shut down the loop once the result was received and in
that case the pipeline would never ever be shut down (and our unit test
would hang).
3 jobs for convert-frame-async-test-deadlock in 7 minutes and 38 seconds (queued for 1 second)
Status Name Job ID Coverage
  Preparation
passed manifest #53907

00:00:23

 
  Build
passed build fedora x86_64 #53908

00:03:40

 
  Full Builds
passed build android arm64 api28 #53909

00:03:34