Allow getting and removing app op modes for non-existent users.

This may happen due to raciness and inconsistent state read from
disks, and the previous app ops implementation allows it, so keep the
same behavior for now.

Bug: 263504888
Test: Fixes system server crashes in ag/20828667 presubmit
Change-Id: I2fbebb909dfef19d1661d850710034ce5ff9f36c