Fix crash in PacManager

The PacManager would clear the pac url by setting it to null, however
everywhere else, pac url is cleared to Uri.EMPTY.  This sometimes leads
to an NPE when PAC is set and cleared rapidly and take down the whole
framework.

Bug: 17581527
Change-Id: I84ce215f4f6a8a7e804372fc0a1e20ac609a21f1