[flexiglass] Show the battery estimate in collapsed view without cutout.

This aligns the implementation with the legacy shade
(QsBatteryModeController.getBatteryMode).

Fix: 414760909
Bug: 298525212
Test: Manually tested by opening the single shade on a device without a
 center cutout, and observing that the battery estimate is shown instead
 of percentage.
Flag: com.android.systemui.scene_container
Change-Id: Ibc50fced41090dbbc573f76e28e339f612b11ee3