Forces AsyncTask to use the main looper

Bug #5333924

This might break applications trying to use AsyncTask on a looper
that is not the main thread, but such apps would already have
issues (AsyncTask has a single static handler and attempting to
use AsyncTask outside of the main thread would likely result
in weird threading issues in other parts of the app.)

Change-Id: Ibbf480627fc7b91326a27d4f5e5af49e8c5b5115