Set device credential's Window flag to be SECURE.

This prevents showing any pattern/pin in recent list.

Fixes: 64339515
Bug: 38258991
Merged-In: I99c283dd4a1f2d579f565ccf5839a705e75ed6fc
Test: visual
Test: runtest --path \
        tests/app/src/com/android/settings/password/ChooseLockGenericTest.java

Change-Id: Ia9935ef710cf1795d9ce464e0a9edff75be9c0c3