Migrate to TaskViewsIterable & TaskViewsIterator - [n/n]

Use TaskViewsIterable to iterate all the current TaskViews
inside `RecentsView.updateGridProperties`. Remove all the indices
usage inside this function as well.

Flag: EXEMPT as no functionality changes
Bug: 379942019
Test: Verified that the TaskViews layout is correct in below scenarios:
      1. Swipe up from an fullscreen app or from home
      2. Dismiss the large tile, a TaskView in the middle either in the
         top row or the bottom row, dismiss the last TaskView.
      3. Clear all TaskViews
      4. Swipe up to the right side of the edge to have a normal
         TaskView as the snapped TaskView
      5. Alt+Tab with more than 6 apps to enter Overview, with a
         hidden TaskView from the alt+tab list as the snapped TaskView.
Change-Id: Ic01534ed03bcc654d5799f43f0cf659d2c86ca31