Mark the stack nullable so IntelliJ will warn about using it without a null check.

Also changes the import to androidx.annotation, which is what we use in BubbleStackView. The default config of IntelliJ seems to ignore android.annotation.Nullable.

Fixes: 133314397
Bug: 133314397
Test: atest SystemUITests
Change-Id: I6887a82787403611ff62cb8bf2e4a2dd9049d5e3