Only auth via FP when device is locked, when Assistant is on top

Fixes: 64586258

Test: 1) swipe FP up 2cm, touch FPS without letting go. device should stay in keyguard but become unlocked
2) launch assistant, touch FPS. No vibration should occur

Test: 1) set up smart lock
2) enter keyguard and make sure smart lock is working (device is unlocked)
3) launch assistant, touch FPS. No vibration should occur

Change-Id: I7177c02c881eee5d6dd2dbabcddc92ec31f8a686