[flexiglass] Disable BackActionInteractor when flexi is on

This class isn't needed when flexi is on.

Bug: 283121968
Test: manually verified that the back gesture and back key_event from
adb both seem to still work to close the shade, QS, or bouncer while
flexi is on
Flag: com.android.systemui.scene_container

Change-Id: I4d66c629037fbecffae5c4bf6848bd1c897d3577