Don't return TYPE_RECENTS task in TopTaskTracker.getCachedTopTask

- We have no use-case to show TYPE_RECENTS (NexusLauncher or RecentsActivity) in Overview, stripping TYPE_RECENTS avoids accidentally showing RecentsAcitivty as a tile on swipe up

Fix: 355263037
Test: manually verify it doesn't break 3P Launcher; the bug itself cannot be verified as it cannot be easily reproduced
Flag: EXEMPT BUG_FIX
Change-Id: Id1b8e43dc7ddb9ad9c781b9a860fe4fba97d4be6