Let `getOffsetToDismissedTask` not rely on `taskCount`

This is a preparation to migrate TaskViews iteration that
rely `getTaskViewCount` to the `TaskViewsIterable` and
`TaskViewsIterator`.

Flag: EXEMPT as no functionality changes
Bug: 379942019
Test: Tested on tangor with `adb shell wm density 480` to simulate
      phone layout, and verified overview layout is correct in
      scenarios below:
      1. Dismiss the first TaskView
      2. Dismiss the last TaskView
      3. Dismiss a TaskView in the middle
      4. Click `ClearAll` button to dismiss all the TaskViews
Change-Id: Icbaf1ea706906097fc6ac60ad3e2ab17d2781284