Migrate using TaskViewCount as the indices

Do not rely on `getTaskViewCount` as the index information.
- Remove the use of the deprecated function `obtain` as well.

Flag: EXEMPT as no functionality changes
Bug: 379942019
Test: Enabled TalkBack on Tangor and entered overview with multiple
      opened apps, verified the information as below:
      1. The number of children pronounced by TalkBack is the number
         of RecentsView children.
      2. Talkback starts from the leftmost visible TaskView, which
         matches the logic that we added the TaskView in reverse order
         to the accessibility children list.
      3. Scrolled inside RecentsView, and checked the from and end
         indices of the a11 event are the same as before.
Change-Id: Ib3c37422ba940aed6c124a8cc0d4732144783d45