Throw exception from SystemUiProxy.getRecentTasks for invalid result

- The reason Recents is empty after a force-stop is when RecentsTasksList.getTasks is called first time after restart, the result is always invalid but we cached the invalid result with a chagneId(=1). The next time we call getTasks again, we think the cached result is valid, and returend an empty list.
- The fix is to mark such result as invalid to avoid caching the wrong result

Fix: 353926204
Test: RecentTasksListTest
Flag: EXEMPT bugfix
Change-Id: If15ab8fd7454db8a08c22b17eaac73f0c78aa75f