[Fingerprint] Don't show warning dialog if screen lock is set up

If the keyguard is already secure (i.e. if the backup screen lock is
already set up), do not show the skip dialog which warns user about
the danger of not having a screen lock.

Bug: 34129157
Test: adb shell am instrument -w com.android.settings.tests.unit
Change-Id: I6f777631487de89ab25a08ea017dd6194dde464d