Use flatMap to ensure NSSL alpha becomes 1f when GONE

When transitioning to OCCLUDED/GONE states, also listen for
statusBarState == SHADE, and when both conditions are met,
emit the shade alpha, which will be 1f. This ensures
HUNs will appear all of the time. Otherwise, let normal
transitions determine alpha.

Fixes: 357316448
Test: atest SharedNotificationContainerViewModelTest
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: I372f5d19cbd1074a2bf29b35afee9f23b50c466c