[Shortcut helper]Moved Recent Apps Shortcuts to System Category

In Shortcut helper, Recent apps shortcuts(cycle forward/back through recent apps)
which were previously under multitasking category is are being moved to System category.

Test: SystemShortcutsSourceTest
Test: MultitaskingShortcutsSourceTest
Flag: EXEMPT Trivial
Fix: 380227478
Change-Id: I3d97e7b3ea96d8fa3b5beee282e0899af3d8e5e9