Support getting AssistContent for recents in Launcher

The goal is to allow Launcher's Recents View to obtain the AssistContent for the recent activity shown in the view. This is accomplished by:
1. Adding a way to pass an AssistDataReceiver to ActivityTaskManager
2. Adding the 'recents' protection level to the GET_TOP_ACTIVITY_INFO permission so that Launcher can call the new method

Bug: 180157890
Test: Manual (tested getting the AssistContent from Launcher recents on local device)
Change-Id: Ibb3d6b68e2b8f22e79a7b6dbbeb1a3e89d2d6a36