Fix null handling in proxies.

ProxyInfo.getPacFileUrl() can not be null.  It will be equal to
Uri.EMPTY.  Checking for null was causing global proxies to never be
disabled.  Or more accurately, global proxies would be disabled, but
would reappear after a reboot.

ProxyInfo.getExclusionListByString() can be null.  If no
exclusion list was specified, the proxy settings would not be
successfully saved, they would disappear after reboot.

Bug: 18453223
Change-Id: I1c27e5dca5b9664bb7468ea909bff489fa110a07