Fixes the system server crash issues caused by null pointer in

NetworkPolicyManagerService.

if the multi user occurs switch and remove then to getPackagesForUid 
the return value will got null,because the user remove is not synced
with the getPackagesForUid,so we must check the return value for 
getPackagesForUid to avoid the crash.

https://code.google.com/p/android/issues/detail?id=211972



Change-Id: If921582da86dbe862f2dad337b0a8d1f20593c99
Signed-off-by: songjinshi <songjinshi@xiaomi.com>