Make detached task trimmable

Otherwise Task#isOnHomeDisplay also returns false for a detached
task, which causes a task without activities is not trimmable
even if it is exclude-from-recent.

Fix: 185481296
Test: atest RecentTasksTest#testVisibleTasks_excludedFromRecents
Change-Id: I66d494376bc750563b914b18e9f4a999913c70e9