Add mechanism for freezing the system recent task list

- In order to support swiping between tasks in a stable list, this allows
  SystemUI to temporarily freeze the task list ordering until either an
  interaction within the current app, or some timeout occurs.

  This does not affect the actual task ordering, only the ordering of the
  tasks via the call to getRecentTasks() made from SystemUI.

Bug: 111926330
Test: atest WmTests:RecentTasksTest

Change-Id: I9ebda02e4e0d34fd2211cfbbed9f7b0ec35f86cc
6 files changed