Start tracking unknownvisibility immediately for launchBehind

Previously, we only started tracking unknownVisibility in
`realStartActivity` which happens after completePause. This is
too late since there can be an arbitrary amount of time before
we get pause and launchBehind apps lifetime shouldn't be
tied to a different task anyways. This CL starts tracking it
on start so that the transition system doesn't interpret it
as syncFinished before we know if it will be visible or not.

Bug: 277759491
Test: Launch home from dream multiple times.
Change-Id: Ifabc089e396717565d351139d761bb967c8946ae