[flexiglass] NullPointerException in QSPanelControllerBaseTest

Several tests in this class failed with a NullPointerException
during its tearDown phase. I couldn't reproduce this locally,
but the stack trace indicates the crash happens within the
LifecycleRegistry when mController.destroy() is called. This
could be caused by an inconsistent lifecycle state when the
ViewLifecycleOwner is destroyed.

This cl fixes the issue by calling onViewDetached() before
destroy() in the test's tearDown method. This can ensure the
view is properly detached and its lifecycle is correctly
handled before the controller is destroyed.

Bug: 430059562
Flag: com.android.systemui.scene_container
Test: com.android.systemui.qs.QSPanelControllerBaseTest
Change-Id: I0b58866cc54f295fe29280951260251d3a427712