[flexiglass] Fix NotificationShelfTest with SceneContainerFlag

When SceneContainerFlag is enabled, NotificationShelf.isAlignedToEnd()
now relies on the internal mAlignedToEnd field (set via
setAlignedToEnd()) instead of primarily using
mAmbientState.getUseSplitShade().

This cl updates the prepareShelfSpy helper in the test to also call
shelfSpy.setAlignedToEnd() to ensure the shelf's alignment state is
correctly configured when SceneContainerFlag is enabled.

Bug: 416716454
Flag: com.android.systemui.scene_container
Test: com.android.systemui.statusbar.notification.stack.NotificationShelfTest
     Locally run this test and verify all the tests are passed with the
     `@EnableSceneContainer` annotation.
Change-Id: Ib3a41fd4aff7414270082eecf7ff035aca6e0b93