Avoid crashing when downloading MitM'd PAC that is too big

There's two pieces to this fix:
1. Move PAC loading off IoThread which isn't meant for
   blocking network fetches.  If the fetch takes more than
   60s Android reboots when the IoThread is used.
2. Limit PAC fetching to 20MB.  Any PAC bigger than that
   is likely evil.
MitM of PACs should only be possbile when a non-SSL PAC URL
is used.

Change-Id: Ie1658a1c705615dc85a7fc68053f0dad8d048294
Fixes: 30100884