Re-enable task trimming

- Skip trimming tasks that are behind the home stack as they can not be
  returned to.
- Adding tag to event log when trimming the task to indicate it was trimmed
  by the recent tasks list
- Update tests to use task builder

Bug: 34270611
Test: Launch task from assistant, ensure that the assistant is not removed
Test: com.android.server.am.RecentTasksTest
Test: #testBackStackTasks_expectNoTrim
Test: #testBehindHomeStackTasks_expectTaskTrimmed
Test: #testOtherDisplayTasks_expectNoTrim
Change-Id: I538c9010f7ce7844bfa93ff3c8900dea6f91bc95