Fix back gesture test.

This changelist disables the glanceable hub back gesture
flag for test exercising legacy behavior disabling this
behavior.

Bug: 382771533
Flag: com.android.systemui.glanceable_hub_back_action
Test: atest QuickStepContractTest#isBackGestureDisabled_hubShowing
Change-Id: I78008eb56c51967c20725bf5e83718a2e82c948b