4/Clean up use of hidden recent task flags

- Remove flags to control which recent tasks are returned -- they are now
  trimmed with the expected behaviour when the task list is modified.
- Remove unused logic related to force-resetting a task based on inactive
  time.
- Remove SysUI logic related to excluded recent tasks since it is already
  handled
- Clean up task list subsetting now that returned list is guaranteed to
  only be visible tasks
- Only use AM interface to fetch recent tasks instead of through AM

Bug: 34270611
Test: runtest --path frameworks/base/services/tests/servicestests/src/com/android/server/am/RecentTasksTest.java
Change-Id: I3f60ce507563152b5704c63985af621b02d0e7c8