[flexiglass] Add a "canSwipeToDismissState" state.

This tracks whether it's currently possible to swipe up to dismiss the
lockscreen without requiring authentication.

Bonus: Fixes a bug in the `isUnlocked` state definition.

Fix: 294982161
Fix: 294984006
Test: new unit tests added, and existing ones modified.

Change-Id: I1fa9d4ca97da737a1bcf386bd27ec3340aed1871