[flexiglass] Fixes back navigation.

Turns out that back navigation was implemented correctly, but the
EdgeBackGestureHandler required the SysUiState to be in certain states
to make the arrow and gesture enabled.

This CL achieves that.

Fix: 289515848
Test: manually verified that back works in the bouncer scene in
Flexiglass

Change-Id: I72b657afc9484569f7883c0a23ec2891a7abeff9