Fix an issue where tasks moving to front would collapse the stack.

ag/18759417 introduced a new call to onTaskMovedToFront which impacted
some bubbles behavior, basically while switching bubbles the
"background" app now appears to move to the front for a moment which
causes bubbles to be collapsed. The fix is to flip the flag for
bubbles switching a bit sooner than we already do.

Test: manual - have some bubbles, visit each of them once (i.e. have
               the activity launched), then visit them all again
             => verify all the bubbles can be visited both times and
               the stack doesn't collapse.
Bug: 235781358
Change-Id: Id0674cafd92d5aeb802a0da77a679996e7041b74