Replace broken UserHandle.SYSTEM check in LPU

ActivityManager.getCurrentUser returns the user currently in the
foreground, not the calling or affected process.

Note:
---
This is just an API for setting the default suggestion when choosing a
new lock type in Settings, so it's not much of a security problem
(background users/profiles could only modify the suggestion, not the
 actual encryption setting).

Change-Id: I98c62be7c5726f99fea0ec9320ad9d85cce850ef