Remove maxScreenId from LauncherProvider and whenever we need
a new screenId, query the database to calculate a new screenId.
Also converted and refactored AddWorkspaceItemsTaskTest
and added some extra test cases.
Test: manual & AddWorkspaceItemsTaskTest.kt
Bug: 199160559
Change-Id: I185f6823fed171d778af0130497f5ffaf89c0a70