[flexiglass] Trigger face unlock when overscrolling bouncer.

When dragging up on the bouncer, there's nowhere for scene navigation to
go; instead, overscroll kicks in and the bouncer UI is translated
slightly upwards. When this happens, trigger a face unlock attempt.

Fix: 299343636
Test: unit test added
Test: manually verified that scrolling upwards on the bouncer triggers
face unlock and actually unlocks the device and transitions to the gone
scene
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I96aa5460821cd81a4a8808348ac633ab736a694b