Set device credential's Window flag to be SECURE.

This prevents showing any pattern/pin in recent list.

Fixes: 64339515
Bug: 38258991
Test: visual
Test: runtest --path \
      tests/app/src/com/android/settings/password/ChooseLockGenericTest.java
(cherry picked from commit 6d479d99)

Change-Id: I99c283dd4a1f2d579f565ccf5839a705e75ed6fc