Quick fix for NPE in onPackageUninstalled() for UPS.

When users are removed and created by tests, there may be a race
condition so that onPackageUninstalled() is called with a user ID that
no longer exists. The call site is hard to fix because it's nested
inside too many layers of package manager logic, but we can at least
detect and skip it in onPackageUninstalled().

Fixes: 282988200
Test: atest CtsDevicePolicyTestCases:android.devicepolicy.cts.UserRestrictionsTest
Change-Id: I719a46eb8675bcb880d60bac0a79fb6714ac8914