[flexiglass] Scene container no longer visible after canceled swipe.

When unlocked, if we swipe down a short distance to reveal the shade
but stop short such that the shade never shows, Flexiglass' scene
container remains visible forever.

This CL fixes that.

Test: unit and integration tests
Test: manually verified that the scene container no longer gets stuck in
this condition
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I5ea7f428635000dc9d1063479c4abab5ce7bef2d