Add nullability and type guarantees

This will help understand how State can be null. Also, make sure that
it's never null in QSTileViewModelAdapter.

Test: manual with both flags on
Test: atest com.android.systemui.qs
Fixes: 382127614
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Flag: com.android.systemui.qs_new_tiles
Change-Id: Ie44dd2dca702253317eb82bba60a397de01f2349