Do not clear critical keys in clear_uid()

If clear_uid() is called on system uid, skip clearing keys with
FLAG_CRITICAL_TO_DEVICE_ENCRYPTION flag since device authenticaion
would be broken without them.

Bug: 34600579
Test: Add device lock under synthtic password, goto Settings/security/encryption,
      tap clear credentials and verify device lock is still intact.

Change-Id: I6c009163831b0901b0973d13906f56139028052c