Fix NPEs in {AppId,Package}AppOpPolicy

We've seen recent NPEs where setting the appop mode fails because the
user doesn't exist in the state. This is due to a race condition so we
need to protect against it.

Fixes: 320267569
Test: Build and boot
Change-Id: I4a8dc1e6370761e61b05bd444bc52e0efec6d59d