Handle activity launch for background users

- When launching activities in background, do not move the
  target stack to front or resume the top activity, it should
  have no observable effect to current user.

- Add the launched background task to recents so that they
  can be found when the background user becomes current.

- Keep track of the last accessed stack for background user,
  so that we know which stack to start with when that user goes
  current

bug: 22929608

Change-Id: I0421a6b490251503df7a7e71465b8aae5138eb2d