Shortcut Helper - Create KeyboardShortcutGroupsSource interface

+ Make system and multitasking shortcut sources implement it. This will
  make conversions easier for the follow up CL.

Test: unit tests in this CL
Bug: 341045652
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite
Change-Id: I6293be479bca97c9ae612a63317c111484fbe048