- 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
45c25ceb3a