1/Clean up RecentTasks list

- Move some logic into the RecentTasks list
- Added some callbacks for future handling of trimming recent tasks based
  on additional policy
- Fix issue when cleaning up tasks where the wrong activity/app info can
  be fetched since the tmp caches were not cleared
- Add some initial tests for existing task list behaviour.

Bug: 34270611
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/am/RecentTasksTest.java

Change-Id: I26730461fa5fb3f5afdabe092438c229381f4f2b