[flexiglass] Fixes SideFpsOverlayViewModel when flexi is on

The sample wasn't working if the showIndicatorForDeviceEntry was
updating dynamically (as it does when flexiglass is on).

This was causing SideFpsOverlayViewModelTest to fail when flexiglass is
on

Bug: 283121968
Test: SideFpsOverlayViewModelTest passes locally now
Flag: com.android.systemui.scene_container
Change-Id: Ieeb5ba6ddd5ef44bca7bf7b432c5d0ab28317caf