Use new onTaskListUpdated() callback

Migrated to general method for receiving updates
whenever the recents list undergoes any additions or
removals.

Test: Opened apps, and as I closed them I ensured
via debugger and log statements that the callback was
being triggered from the framework module. See
tests in RecentTasksTest

fixes: 111077107
Change-Id: Ia9bddb50861a1b107e6a88c9f9bb89944800d5d8