[flexiglass] Fix back gesture not working in Bouncer

The BackHandler in AlternateBouncer always remains registered. Since it is high up in the compose hierarchy, it takes precedence over other BackHandlers, stealing back gesture events from them.

To fix this, we should disable it when AlternateBouncer is not visible.

Bug: 359950520
Bug: 353955910
Test: Manual, i.e. verify that back gesture on Bouncer works
Flag: com.android.systemui.scene_container
Change-Id: I3339c793d6937a51f41c9d15793d3903725c73e7